--- a/browser/components/customizableui/PanelMultiView.jsm
+++ b/browser/components/customizableui/PanelMultiView.jsm
@@ -863,50 +863,49 @@ var PanelMultiView = class extends this.
     let viewRect;
     if (reverse) {
       // Use the cached size when going back to a previous view, but not when
       // reopening a subview, because its contents may have changed.
       viewRect = { width: nextPanelView.knownWidth,
                    height: nextPanelView.knownHeight };
       nextPanelView.visible = true;
     } else if (viewNode.customRectGetter) {
-      // Can't use Object.assign directly with a DOM Rect object because its properties
-      // aren't enumerable.
+      // We use a customRectGetter for WebExtensions panels, because they need
+      // to query the size from an embedded browser. The presence of this
+      // getter also provides an indication that the view node shouldn't be
+      // moved around, otherwise the state of the browser would get disrupted.
       let width = prevPanelView.knownWidth;
       let height = prevPanelView.knownHeight;
       viewRect = Object.assign({height, width}, viewNode.customRectGetter());
       let header = viewNode.firstChild;
       if (header && header.classList.contains("panel-header")) {
         viewRect.height += this._dwu.getBoundsWithoutFlushing(header).height;
       }
       nextPanelView.visible = true;
       nextPanelView.descriptionHeightWorkaround();
     } else {
-      let oldSibling = viewNode.nextSibling || null;
       this._offscreenViewStack.style.minHeight = olderView.knownHeight + "px";
       this._offscreenViewStack.appendChild(viewNode);
       nextPanelView.visible = true;
 
       // Now that the subview is visible, we can check the height of the
       // description elements it contains.
       nextPanelView.descriptionHeightWorkaround();
 
       viewRect = await window.promiseDocumentFlushed(() => {
         return this._dwu.getBoundsWithoutFlushing(viewNode);
       });
       // Bail out if the panel was closed in the meantime.
       if (!nextPanelView.isOpenIn(this)) {
         return;
       }
 
-      try {
-        this._viewStack.insertBefore(viewNode, oldSibling);
-      } catch (ex) {
-        this._viewStack.appendChild(viewNode);
-      }
+      // Place back the view after all the other views that are already open in
+      // order for the transition to work as expected.
+      this._viewStack.appendChild(viewNode);
 
       this._offscreenViewStack.style.removeProperty("min-height");
     }
 
     this._transitioning = true;
     details.phase = TRANSITION_PHASES.PREPARE;
 
     // The 'magic' part: build up the amount of pixels to move right or left.