diff --git a/toolkit/components/contextualidentity/ContextualIdentityService.sys.mjs b/toolkit/components/contextualidentity/ContextualIdentityService.sys.mjs
index 5702ff28cc22206f5ce16584dac8a78d816562ce..3d08ecc97ce5995b30d8a4af0c33df329b428008 100644
--- a/toolkit/components/contextualidentity/ContextualIdentityService.sys.mjs
+++ b/toolkit/components/contextualidentity/ContextualIdentityService.sys.mjs
@@ -270,11 +270,29 @@ _ContextualIdentityService.prototype = {
     });
   },
 
-  create(name, icon, color) {
+  create(name, icon, color, id = null) {
     this.ensureDataReady();
 
-    // Retrieve the next userContextId available.
-    let userContextId = ++this._lastUserContextId;
+    if (id !== null) {
+      id = typeof id == "string" ? Number(id) : id;
+      if (!Number.isSafeInteger(id) || id <= 0) {
+        throw new Error(`Invalid userContextId '${id}'`);
+      }
+    }
+
+    let userContextId;
+    if (
+      id !== null &&
+      !this._identities.some(identity => identity.userContextId === id)
+    ) {
+      userContextId = id;
+      this._lastUserContextId = Math.max(
+        this._lastUserContextId,
+        userContextId
+      );
+    } else {
+      userContextId = ++this._lastUserContextId;
+    }
 
     // Throw an error if the next userContextId available is invalid (the one associated to
     // MAX_USER_CONTEXT_ID is already reserved to "userContextIdInternal.webextStorageLocal", which