chore: fix issue where transparent browsers have a forced-white sync, p=#12392

* chore: fix issue where transparent browsers have a forced-white sync background without breaking non-transparent browsers

* chore: fix linting issue

src/zen/sessionstore/ZenWindowSync.sys.mjs

@@ -798,6 +798,7 @@ class nsZenWindowSync {
           {
             fullScale: true,
             fullViewport: true,
+            backgroundColor: "transparent",
           }
         );