diff --git a/browser/components/tabbrowser/content/tabs.js b/browser/components/tabbrowser/content/tabs.js
index 0fbdbf3aefc467880e6b0bae2615cb145735cb0f..bf956639f7d17d6d652f3b2d6cb174904b5b5da9 100644
--- a/browser/components/tabbrowser/content/tabs.js
+++ b/browser/components/tabbrowser/content/tabs.js
@@ -93,7 +93,7 @@
       };
       this.arrowScrollbox._canScrollToElement = element => {
         if (isTab(element)) {
-          return !element.pinned || !this.hasAttribute("positionpinnedtabs");
+          return !element.hasAttribute("zen-essential") || !this.hasAttribute("positionpinnedtabs");
         }
         return true;
       };
@@ -347,7 +347,7 @@
       // and we're not hitting the scroll buttons.
       if (
         event.button != 0 ||
-        event.target != this.arrowScrollbox ||
+        event.target != document.getElementById("zen-tabs-wrapper") ||
         event.composedTarget.localName == "toolbarbutton"
       ) {
         return;
@@ -396,6 +396,7 @@
             // Reset the "ignored click" flag
             target._ignoredCloseButtonClicks = false;
           }
+          gZenUIManager.saveScrollbarState();
         }
 
         /* Protects from close-tab-button errant doubleclick:
@@ -693,7 +694,7 @@
       if (this.#isContainerVerticalPinnedExpanded(tab)) {
         // In expanded vertical mode, the max number of pinned tabs per row is dynamic
         // Set this before adjusting dragged tab's position
-        let pinnedTabs = this.visibleTabs.slice(0, gBrowser.pinnedTabCount);
+        let pinnedTabs = this.visibleTabs.filter(tab => !tab.hasAttribute("zen-glance-tab")).slice(0, gBrowser._numZenVisibleEssentials);
         let tabsPerRow = 0;
         let position = 0;
         for (let pinnedTab of pinnedTabs) {
@@ -899,6 +900,10 @@
       }
 
       let draggedTab = event.dataTransfer.mozGetDataAt(TAB_DROP_TYPE, 0);
+      if (draggedTab && effects === "move") {
+        gZenPinnedTabManager.applyDragoverClass(event, draggedTab);
+        gZenViewSplitter.onBrowserDragEndToSplit(event);
+      }
       if (
         (effects == "move" || effects == "copy") &&
         this == draggedTab.container &&
@@ -1012,6 +1017,18 @@
 
       this._tabDropIndicator.hidden = true;
       event.stopPropagation();
+      if (draggedTab?.hasAttribute("zen-has-splitted")) {
+        draggedTab.removeAttribute("zen-has-splitted");
+        draggedTab._visuallySelected = false;
+      }
+      if (draggedTab && dropEffect == "move") {
+        let moved = gZenPinnedTabManager.moveToAnotherTabContainerIfNecessary(event, movingTabs);
+
+        if (moved) {
+          this._finishMoveTogetherSelectedTabs(draggedTab);
+          return;
+        }
+      }
       if (draggedTab && dropEffect == "copy") {
         // copy the dropped tab (wherever it's from)
         let newIndex = this._getDropIndex(event);
@@ -1050,10 +1067,11 @@
           }
         } else {
           let isPinned = draggedTab.pinned;
-          let numPinned = gBrowser.pinnedTabCount;
-          let tabs = this.ariaFocusableItems.slice(
-            isPinned ? 0 : numPinned,
-            isPinned ? numPinned : undefined
+          let numPinned = gBrowser._numVisiblePinTabsWithoutCollapsed;
+          let essential = draggedTab.hasAttribute("zen-essential");
+          let tabs = this.ariaFocusableItems.filter(tab => !tab.hasAttribute("zen-glance-tab")).slice(
+            isPinned ? (essential ? 0 : gBrowser._numZenVisibleEssentials) : numPinned,
+            isPinned ? (essential ? gBrowser._numZenVisibleEssentials : numPinned) : undefined
           );
           let size = this.verticalMode ? "height" : "width";
           let screenAxis = this.verticalMode ? "screenY" : "screenX";
@@ -1129,7 +1147,7 @@
                 tab.removeAttribute("tabdrop-samewindow");
                 resolve();
               };
-              if (gReduceMotion) {
+              if (true || gReduceMotion) {
                 postTransitionCleanup();
               } else {
                 let onTransitionEnd = transitionendEvent => {
@@ -1246,7 +1264,7 @@
               return;
             }
           }
-
+          if (gZenViewSplitter.handleTabDrop(event, urls, replace, inBackground)) return;
           gBrowser.loadTabs(urls, {
             inBackground,
             replace,
@@ -1276,13 +1294,23 @@
         return;
       }
 
-      this._finishMoveTogetherSelectedTabs(draggedTab);
       this._finishAnimateTabMove();
 
+      if (!dt.mozUserCancelled && dt.dropEffect == "none" && !this._isCustomizing) {
+        const moved = gZenViewSplitter.moveTabToSplitView(event, draggedTab);
+        if (moved) {
+          delete draggedTab._dragData;
+          return;
+        }
+      }
+
+      this._finishMoveTogetherSelectedTabs(draggedTab);
+
       if (
         dt.mozUserCancelled ||
         dt.dropEffect != "none" ||
-        this._isCustomizing
+        this._isCustomizing ||
+        draggedTab.pinned
       ) {
         delete draggedTab._dragData;
         return;
@@ -1504,7 +1532,7 @@
     }
 
     get newTabButton() {
-      return this.querySelector("#tabs-newtab-button");
+      return ZenWorkspaces.activeWorkspaceStrip.querySelector("#tabs-newtab-button");
     }
 
     get verticalMode() {
@@ -1524,29 +1552,41 @@
       if (this.#allTabs) {
         return this.#allTabs;
       }
-      let children = Array.from(this.arrowScrollbox.children);
+      let children = ZenWorkspaces.tabboxChildren;
       // remove arrowScrollbox periphery element
       children.pop();
 
       // explode tab groups
       // Iterate backwards over the array to preserve indices while we modify
       // things in place
-      for (let i = children.length - 1; i >= 0; i--) {
-        if (children[i].tagName == "tab-group") {
-          children.splice(i, 1, ...children[i].tabs);
-        }
-      }
 
       this.#allTabs = [
-        ...this.verticalPinnedTabsContainer.children,
+        ...document.getElementById("zen-essentials-container").children, ...this.verticalPinnedTabsContainer.children,
         ...children,
       ];
+      const lastPinnedTabIdx = gBrowser.pinnedTabCount;
+      for (let i = 0; i < this.#allTabs.length; i++) {
+        // add glance tabs (tabs inside tabs) to the list
+        const glanceTab = this.#allTabs[i].querySelector("tab[zen-glance-tab]");
+        if (glanceTab) {
+          // insert right after the parent tab. note: it must be inserted before
+          // the last pinned tab so it can be inserted in the correct order
+          this.#allTabs.splice(Math.max(i + 1, lastPinnedTabIdx), 0, glanceTab);
+          i++;
+        } else if (this.#allTabs[i].classList.contains("vertical-pinned-tabs-container-separator")) {
+          // remove the separator from the list
+          this.#allTabs.splice(i, 1);
+          i--;
+        } else if (this.#allTabs[i].tagName == "tab-group") {
+          this.#allTabs.splice(i, 1, ...this.#allTabs[i].tabs);
+        }
+      }
       return this.#allTabs;
     }
 
     get allGroups() {
       let children = Array.from(this.arrowScrollbox.children);
-      return children.filter(node => node.tagName == "tab-group");
+      return ZenWorkspaces.allTabGroups;
     }
 
     /**
@@ -1566,7 +1606,7 @@
      */
     get visibleTabs() {
       if (!this.#visibleTabs) {
-        this.#visibleTabs = this.openTabs.filter(tab => tab.visible);
+        this.#visibleTabs = this.openTabs.filter(tab => tab.visible && !tab.hasAttribute("zen-empty-tab"));
       }
       return this.#visibleTabs;
     }
@@ -1601,23 +1641,18 @@
       }
 
       let elementIndex = 0;
-      let verticalPinnedTabsContainer = document.getElementById(
-        "vertical-pinned-tabs-container"
-      );
-      for (let i = 0; i < verticalPinnedTabsContainer.childElementCount; i++) {
-        verticalPinnedTabsContainer.children[i].elementIndex = elementIndex++;
-      }
-      let children = Array.from(this.arrowScrollbox.children);
+      let verticalPinnedTabsContainer = this.verticalPinnedTabsContainer
+      let children = ZenWorkspaces.tabboxChildrenWithoutEmpty;
 
       let focusableItems = [];
-      for (let child of children) {
+      for (let child of [...document.getElementById("zen-essentials-container").children, ...verticalPinnedTabsContainer.children, ...children]) {
         if (isTab(child) && child.visible) {
           child.elementIndex = elementIndex++;
           focusableItems.push(child);
         } else if (isTabGroup(child)) {
           child.labelElement.elementIndex = elementIndex++;
           focusableItems.push(child.labelElement);
-          if (!child.collapsed) {
+          if (!child.collapsed && !child.hasAttribute("split-view-group")) {
             let visibleTabsInGroup = child.tabs.filter(tab => tab.visible);
             visibleTabsInGroup.forEach(tab => {
               tab.elementIndex = elementIndex++;
@@ -1627,10 +1662,7 @@
         }
       }
 
-      this.#focusableItems = [
-        ...verticalPinnedTabsContainer.children,
-        ...focusableItems,
-      ];
+      this.#focusableItems = focusableItems;
 
       return this.#focusableItems;
     }
@@ -1638,6 +1670,7 @@
     _invalidateCachedTabs() {
       this.#allTabs = null;
       this._invalidateCachedVisibleTabs();
+      ZenWorkspaces._allStoredTabs = null;
     }
 
     _invalidateCachedVisibleTabs() {
@@ -1652,8 +1685,8 @@
     #isContainerVerticalPinnedExpanded(tab) {
       return (
         this.verticalMode &&
-        tab.hasAttribute("pinned") &&
-        this.hasAttribute("expanded")
+        (tab.hasAttribute("zen-essential")) &&
+        (this.hasAttribute("expanded") || document.documentElement.hasAttribute("zen-sidebar-expanded"))
       );
     }
 
@@ -1668,7 +1701,7 @@
 
       if (node == null) {
         // We have a container for non-tab elements at the end of the scrollbox.
-        node = this.arrowScrollbox.lastChild;
+        node = ZenWorkspaces.activeWorkspaceStrip.lastChild;
       }
 
       node.before(tab);
@@ -1763,7 +1796,7 @@
           // There are separate "new tab" buttons for horizontal tabs toolbar, vertical tabs and
           // for when the tab strip is overflowed (which is shared by vertical and horizontal tabs);
           // Attach the long click popup to all of them.
-          const newTab = document.getElementById("new-tab-button");
+          const newTab = ZenWorkspaces.activeWorkspaceStrip.querySelector("#tabs-newtab-button");
           const newTab2 = this.newTabButton;
           const newTabVertical = document.getElementById(
             "vertical-tabs-newtab-button"
@@ -1846,7 +1879,7 @@
           let rect = ele => {
             return window.windowUtils.getBoundsWithoutFlushing(ele);
           };
-          let tab = this.visibleTabs[gBrowser.pinnedTabCount];
+          let tab = this.visibleTabs[gBrowser._numVisiblePinTabs];
           if (tab && rect(tab).width <= this._tabClipWidth) {
             this.setAttribute("closebuttons", "activetab");
           } else {
@@ -1858,10 +1891,12 @@
 
     _handleTabSelect(aInstant) {
       let selectedTab = this.selectedItem;
+      if (!selectedTab) return;
       if (this.overflowing) {
         this.arrowScrollbox.ensureElementIsVisible(selectedTab, aInstant);
       }
 
+      gZenCompactModeManager.flashSidebarIfNecessary(aInstant);
       selectedTab._notselectedsinceload = false;
     }
 
@@ -1873,7 +1908,7 @@
         return;
       }
 
-      let tabs = this.visibleTabs;
+      let tabs = this.visibleTabs.filter(tab => !tab.hasAttribute("zen-glance-tab"));
       if (!tabs.length) {
         return;
       }
@@ -1909,7 +1944,7 @@
         if (isEndTab && !this._hasTabTempMaxWidth) {
           return;
         }
-        let numPinned = gBrowser.pinnedTabCount;
+        let numPinned = gBrowser._numVisiblePinTabs;
         // Force tabs to stay the same width, unless we're closing the last tab,
         // which case we need to let them expand just enough so that the overall
         // tabbar width is the same.
@@ -1924,7 +1959,7 @@
         let tabsToReset = [];
         for (let i = numPinned; i < tabs.length; i++) {
           let tab = tabs[i];
-          tab.style.setProperty("max-width", aTabWidth, "important");
+          //tab.style.setProperty("max-width", aTabWidth, "important");
           if (!isEndTab) {
             // keep tabs the same width
             tab.style.transition = "none";
@@ -1990,16 +2025,15 @@
       // Move pinned tabs to another container when the tabstrip is toggled to vertical
       // and when session restore code calls _positionPinnedTabs; update styling whenever
       // the number of pinned tabs changes.
-      let verticalTabsContainer = document.getElementById(
-        "vertical-pinned-tabs-container"
-      );
+      let verticalTabsContainer = this.verticalPinnedTabsContainer;
       let numPinned = gBrowser.pinnedTabCount;
-
-      if (gBrowser.pinnedTabCount !== verticalTabsContainer.children.length) {
-        let tabs = this.visibleTabs;
+      if (!ZenWorkspaces._hasInitializedTabsStrip) return;
+      let count = ZenWorkspaces.makeSurePinTabIsInCorrectPosition();
+      if (gBrowser.pinnedTabCount !== (verticalTabsContainer.children.length - count - 1 + document.getElementById("zen-essentials-container").children.length)) {
+        let tabs = this.allTabs.filter(tab => !tab.hasAttribute("zen-glance-tab"));
         for (let i = 0; i < numPinned; i++) {
           tabs[i].style.marginInlineStart = "";
-          verticalTabsContainer.appendChild(tabs[i]);
+          tabs[i].hasAttribute("zen-essential") ? document.getElementById("zen-essentials-container").appendChild(tabs[i].group?.hasAttribute("split-view-group") ? tabs[i].group : tabs[i]) : verticalTabsContainer.insertBefore(tabs[i].group?.hasAttribute("split-view-group") ? tabs[i].group : tabs[i], verticalTabsContainer.lastChild);
         }
       }
 
@@ -2007,9 +2041,7 @@
     }
 
     _resetVerticalPinnedTabs() {
-      let verticalTabsContainer = document.getElementById(
-        "vertical-pinned-tabs-container"
-      );
+      let verticalTabsContainer = this.verticalPinnedTabsContainer;
 
       if (!verticalTabsContainer.children.length) {
         return;
@@ -2022,8 +2054,8 @@
     }
 
     _positionPinnedTabs() {
-      let tabs = this.visibleTabs;
-      let numPinned = gBrowser.pinnedTabCount;
+      let tabs = this.visibleTabs.filter(tab => !tab.hasAttribute("zen-glance-tab"));
+      let numPinned = gBrowser._numVisiblePinTabs;
       let absPositionHorizontalTabs =
         this.overflowing && tabs.length > numPinned && numPinned > 0;
 
@@ -2032,7 +2064,7 @@
 
       if (this.verticalMode) {
         this._updateVerticalPinnedTabs();
-      } else if (absPositionHorizontalTabs) {
+      } else if (absPositionHorizontalTabs && false) {
         let layoutData = this._pinnedTabsLayoutCache;
         let uiDensity = document.documentElement.getAttribute("uidensity");
         if (!layoutData || layoutData.uiDensity != uiDensity) {
@@ -2104,7 +2136,7 @@
         return;
       }
 
-      let tabs = this.visibleTabs.slice(0, gBrowser.pinnedTabCount);
+      let tabs = this.visibleTabs.filter(tab => !tab.hasAttribute("zen-glance-tab")).slice(0, gBrowser._numZenVisibleEssentials);
 
       let directionX = screenX > dragData.animLastScreenX;
       let directionY = screenY > dragData.animLastScreenY;
@@ -2112,7 +2144,7 @@
       dragData.animLastScreenX = screenX;
 
       let { width: tabWidth, height: tabHeight } =
-        draggedTab.getBoundingClientRect();
+        (draggedTab.group?.hasAttribute("split-view-group") ? draggedTab.group : draggedTab).getBoundingClientRect();
       let shiftSizeX = tabWidth * movingTabs.length;
       let shiftSizeY = tabHeight;
       dragData.tabWidth = tabWidth;
@@ -2296,11 +2328,15 @@
       this.#clearDragOverCreateGroupTimer();
 
       let isPinned = draggedTab.pinned;
-      let numPinned = gBrowser.pinnedTabCount;
-      let tabs = this.ariaFocusableItems.slice(
-        isPinned ? 0 : numPinned,
-        isPinned ? numPinned : undefined
+      let numPinned = gBrowser._numVisiblePinTabsWithoutCollapsed;
+      let essential = draggedTab.hasAttribute("zen-essential");
+      let tabs = this.ariaFocusableItems.filter(tab => !tab.hasAttribute("zen-glance-tab")).slice(
+        isPinned ? (essential ? 0 : gBrowser._numZenVisibleEssentials) : numPinned,
+        isPinned ? (essential ? gBrowser._numZenVisibleEssentials : numPinned) : undefined
       );
+      if (draggedTab.group?.hasAttribute("split-view-group")) {
+        draggedTab = draggedTab.group.labelElement;
+      }
 
       if (this.#rtlMode) {
         tabs.reverse();
@@ -2328,6 +2364,12 @@
       let lastTab = tabs.at(-1);
       let lastMovingTab = movingTabs.at(-1);
       let firstMovingTab = movingTabs[0];
+      if (lastMovingTab.group?.hasAttribute("split-view-group")) {
+        lastMovingTab = lastMovingTab.group;
+      }
+      if (firstMovingTab.group?.hasAttribute("split-view-group")) {
+        firstMovingTab = firstMovingTab.group;
+      }
       let endEdge = ele => ele[screenAxis] + bounds(ele)[size];
       let lastMovingTabScreen = endEdge(lastMovingTab);
       let firstMovingTabScreen = firstMovingTab[screenAxis];
@@ -2348,7 +2390,11 @@
       translate = Math.min(Math.max(translate, firstBound), lastBound);
 
       for (let tab of movingTabs) {
-        tab.style.transform = `${translateAxis}(${translate}px)`;
+        if (tab.group?.hasAttribute("split-view-group")) {
+          tab.group.style.transform = `${translateAxis}(${translate}px)`;
+        } else {
+          tab.style.transform = `${translateAxis}(${translate}px)`;
+        }
       }
 
       dragData.translatePos = translate;
@@ -2379,7 +2425,7 @@
       let getTabShift = (item, dropElementIndex) => {
         if (
           item.elementIndex < draggedTab.elementIndex &&
-          item.elementIndex >= dropElementIndex
+          item.elementIndex + !!item.closest("tab-group")?.hasAttribute("split-view-group") >= dropElementIndex
         ) {
           return this.#rtlMode ? -shiftSize : shiftSize;
         }
@@ -2485,6 +2531,9 @@
             break;
           }
           let element = tabs[mid];
+          if (element?.group?.hasAttribute("split-view-group")) {
+            element = element.group.labelElement;
+          }
           let elementForSize = isTabGroupLabel(element)
             ? element.parentElement
             : element;
@@ -2507,6 +2556,10 @@
       if (!dropElement) {
         dropElement = this.ariaFocusableItems[oldDropElementIndex];
       }
+      if (dropElement?.group?.hasAttribute("split-view-group")) {
+        // We focus the group label element, not the group itself.
+        dropElement = dropElement.group.labelElement;
+      }
       let newDropElementIndex = dropElement
         ? dropElement.elementIndex
         : oldDropElementIndex;
@@ -2566,12 +2619,12 @@
         }
       }
 
-      if (gBrowser._tabGroupsEnabled && !isPinned) {
+      if (true) {
         let dragOverGroupingThreshold = 1 - moveOverThreshold;
 
         // When dragging tab(s) over an ungrouped tab, signal to the user
         // that dropping the tab(s) will create a new tab group.
-        shouldCreateGroupOnDrop =
+        shouldCreateGroupOnDrop = false &&
           dropElement != draggedTab &&
           isTab(dropElement) &&
           !dropElement?.group &&
@@ -2639,7 +2692,7 @@
       // Shift background tabs to leave a gap where the dragged tab
       // would currently be dropped.
       for (let item of tabs) {
-        if (item == draggedTab) {
+        if (item == draggedTab || (item.group?.hasAttribute("split-view-group") && item.group == draggedTab.group)) {
           continue;
         }
 
@@ -2648,6 +2701,9 @@
         if (isTabGroupLabel(item)) {
           // Shift the `.tab-group-label-container` to shift the label element.
           item = item.parentElement;
+          if (item.parentElement?.hasAttribute("split-view-group")) {
+            item = item.parentElement;
+          }
         }
         item.style.transform = transform;
       }
@@ -2697,8 +2753,9 @@
       );
     }
 
-    _finishAnimateTabMove() {
-      if (!this.hasAttribute("movingtab")) {
+    _finishAnimateTabMove(always = false) {
+      gZenPinnedTabManager.removeTabContainersDragoverClass();
+      if (!this.hasAttribute("movingtab") && !always) {
         return;
       }
 
@@ -2711,6 +2768,12 @@
           item = item.parentElement;
         }
         item.style.transform = "";
+        if (item.closest("tab-group")?.hasAttribute("split-view-group")) item.closest("tab-group").style.transform = "";
+        if (item.closest("tab-group")?.hasAttribute("split-view-group")) {
+          for (let tab of item.closest("tab-group").tabs) {
+            tab.style.transform = "";
+          }
+        }
         item.removeAttribute("dragover-createGroup");
       }
       this.removeAttribute("movingtab-createGroup");
@@ -2754,7 +2817,7 @@
         let postTransitionCleanup = () => {
           movingTab._moveTogetherSelectedTabsData.animate = false;
         };
-        if (gReduceMotion) {
+        if (true || gReduceMotion) {
           postTransitionCleanup();
         } else {
           let onTransitionEnd = transitionendEvent => {
@@ -2924,7 +2987,7 @@
     }
 
     _notifyBackgroundTab(aTab) {
-      if (aTab.pinned || !aTab.visible || !this.overflowing) {
+      if (aTab.hasAttribute("zen-essential") || !aTab.visible || !this.overflowing) {
         return;
       }