--- a/browser/base/content/browser.css
+++ b/browser/base/content/browser.css
@@ -108,17 +108,17 @@ panelview {
   transition: transform var(--panelui-subview-transition-duration);
 }
 
 panelview:not([mainview]):not([current]) {
   transition: visibility 0s linear var(--panelui-subview-transition-duration);
   visibility: collapse;
 }
 
-panelview:not([title]) > .panel-header {
+panelview[mainview] > .panel-header {
   display: none;
 }
 
 tabbrowser {
   -moz-binding: url("chrome://browser/content/tabbrowser.xml#tabbrowser");
 }
 
 .tabbrowser-tabs {

--- a/browser/base/content/browser.xul
+++ b/browser/base/content/browser.xul
@@ -606,16 +606,17 @@
                 accesskey="&selectAllCmd.accesskey;"
                 cmd="cmd_selectAll"/>
       <menuseparator/>
       <menuitem label="&syncSyncNowItem.label;"
                 accesskey="&syncSyncNowItem.accesskey;"
                 id="syncedTabsRefreshFilter"/>
     </menupopup>
   </popupset>
+  <box id="appMenu-viewCache" hidden="true"/>
 
 #ifdef CAN_DRAW_IN_TITLEBAR
 <vbox id="titlebar">
   <hbox id="titlebar-content">
     <spacer id="titlebar-spacer" flex="1"/>
     <hbox id="titlebar-buttonbox-container">
 #ifdef XP_WIN
       <hbox id="private-browsing-indicator-titlebar">

--- a/browser/components/customizableui/PanelMultiView.jsm
+++ b/browser/components/customizableui/PanelMultiView.jsm
@@ -242,16 +242,21 @@ this.PanelMultiView = class {
       document.getAnonymousElementByAttribute(this.node, "anonid", "viewContainer");
     this._mainViewContainer =
       document.getAnonymousElementByAttribute(this.node, "anonid", "mainViewContainer");
     this._subViews =
       document.getAnonymousElementByAttribute(this.node, "anonid", "subViews");
     this._viewStack =
       document.getAnonymousElementByAttribute(this.node, "anonid", "viewStack");
 
+    XPCOMUtils.defineLazyGetter(this, "_panelViewCache", () => {
+      let viewCacheId = this.node.getAttribute("viewCacheId");
+      return viewCacheId ? document.getElementById(viewCacheId) : null;
+    });
+
     this._panel.addEventListener("popupshowing", this);
     this._panel.addEventListener("popuphidden", this);
     this._panel.addEventListener("popupshown", this);
     if (this.panelViews) {
       let cs = window.getComputedStyle(document.documentElement);
       // Set CSS-determined attributes now to prevent a layout flush when we do
       // it when transitioning between panels.
       this._dir = cs.direction;
@@ -283,47 +288,76 @@ this.PanelMultiView = class {
       Object.defineProperty(this.node, method, {
         enumerable: true,
         value: (...args) => this[method](...args)
       });
     });
   }
 
   destructor() {
+    // Guard against re-entrancy.
+    if (!this.node)
+      return;
+
     if (this._mainView) {
-      this._mainView.removeAttribute("mainview");
+      let mainView = this._mainView;
+      if (this._panelViewCache)
+        this._panelViewCache.appendChild(mainView);
+      mainView.removeAttribute("mainview");
     }
+    if (this._subViews)
+      this._moveOutKids(this._subViews);
+
     if (this.panelViews) {
+      this._moveOutKids(this._viewStack);
       this.panelViews.clear();
     } else {
       this._clickCapturer.removeEventListener("click", this);
     }
     this._panel.removeEventListener("popupshowing", this);
     this._panel.removeEventListener("popupshown", this);
     this._panel.removeEventListener("popuphidden", this);
     this.node = this._clickCapturer = this._viewContainer = this._mainViewContainer =
-      this._subViews = this._viewStack = this.__dwu = null;
+      this._subViews = this._viewStack = this.__dwu = this._panelViewCache = null;
+  }
+
+  /**
+   * Remove any child subviews into the panelViewCache, to ensure
+   * they remain usable even if this panelmultiview instance is removed
+   * from the DOM.
+   * @param viewNodeContainer the container from which to remove subviews
+   */
+  _moveOutKids(viewNodeContainer) {
+    if (!this._panelViewCache)
+      return;
+
+    // Node.children and Node.childNodes is live to DOM changes like the
+    // ones we're about to do, so iterate over a static copy:
+    let subviews = Array.from(viewNodeContainer.childNodes);
+    for (let subview of subviews) {
+      // XBL lists the 'children' XBL element explicitly. :-(
+      if (subview.nodeName != "children")
+        this._panelViewCache.appendChild(subview);
+    }
   }
 
   goBack(target) {
     let [current, previous] = this.panelViews.back();
     return this.showSubView(current, target, previous);
   }
 
   /**
-   * Checks whether it is possible to navigate backwards currently.
-   * Since the visibility of the back button is dependent - right now - on the
-   * fact that there's a view title set, we use that heuristic to determine this
-   * capability.
+   * Checks whether it is possible to navigate backwards currently. Returns
+   * false if this is the panelmultiview's mainview, true otherwise.
    *
    * @param  {panelview} view View to check, defaults to the currently active view.
    * @return {Boolean}
    */
   _canGoBack(view = this._currentSubView) {
-    return !!view.getAttribute("title");
+    return view != this._mainView;
   }
 
   setMainView(aNewMainView) {
     if (this._mainView) {
       if (!this.panelViews)
         this._subViews.appendChild(this._mainView);
       this._mainView.removeAttribute("mainview");
     }
@@ -355,17 +389,17 @@ this.PanelMultiView = class {
           this.node.setAttribute("viewtype", "main");
         });
       }
 
       this._shiftMainView();
     }
   }
 
-  showSubView(aViewId, aAnchor, aPreviousView, aAdopted = false) {
+  showSubView(aViewId, aAnchor, aPreviousView) {
     const {document, window} = this;
     return (async () => {
       // Support passing in the node directly.
       let viewNode = typeof aViewId == "string" ? this.node.querySelector("#" + aViewId) : aViewId;
       if (!viewNode) {
         viewNode = document.getElementById(aViewId);
         if (viewNode) {
           if (this.panelViews) {
@@ -410,17 +444,17 @@ this.PanelMultiView = class {
       let detail = {
         blockers: new Set(),
         addBlocker(aPromise) {
           this.blockers.add(aPromise);
         },
       };
 
       // Make sure that new panels always have a title set.
-      if (this.panelViews && aAdopted && aAnchor) {
+      if (this.panelViews && aAnchor) {
         if (aAnchor && !viewNode.hasAttribute("title"))
           viewNode.setAttribute("title", aAnchor.getAttribute("label"));
         viewNode.classList.add("PanelUI-subView");
         let custWidget = CustomizableWidgets.find(widget => widget.viewId == viewNode.id);
         if (custWidget) {
           if (custWidget.onInit)
             custWidget.onInit(aAnchor);
           custWidget.onViewShowing({ target: aAnchor, detail });

--- a/browser/components/customizableui/content/panelUI.inc.xul
+++ b/browser/components/customizableui/content/panelUI.inc.xul
@@ -4,17 +4,18 @@
 
 <panel id="PanelUI-popup"
        role="group"
        type="arrow"
        hidden="true"
        flip="slide"
        position="bottomcenter topright"
        noautofocus="true">
-  <panelmultiview id="PanelUI-multiView" mainViewId="PanelUI-mainView">
+  <panelmultiview id="PanelUI-multiView" mainViewId="PanelUI-mainView"
+                  viewCacheId="appMenu-viewCache">
     <panelview id="PanelUI-mainView" context="customizationPanelContextMenu">
       <vbox id="PanelUI-contents-scroller">
         <vbox id="PanelUI-contents" class="panelUI-grid"/>
       </vbox>
 
       <footer id="PanelUI-footer">
         <vbox id="PanelUI-footer-addons"></vbox>
         <toolbarbutton class="panel-banner-item"
@@ -498,17 +499,19 @@
 <panel id="appMenu-popup"
        class="cui-widget-panel"
        role="group"
        type="arrow"
        hidden="true"
        flip="slide"
        position="bottomcenter topright"
        noautofocus="true">
-  <photonpanelmultiview id="appMenu-multiView" mainViewId="appMenu-mainView" descriptionheightworkaround="true">
+  <photonpanelmultiview id="appMenu-multiView" mainViewId="appMenu-mainView"
+                        descriptionheightworkaround="true"
+                        viewCacheId="appMenu-viewCache">
     <panelview id="appMenu-mainView" class="PanelUI-subView">
       <vbox class="panel-subview-body">
         <vbox id="appMenu-addon-banners"/>
         <toolbarbutton class="panel-banner-item"
                        label-update-available="&updateAvailable.panelUI.label;"
                        label-update-manual="&updateManual.panelUI.label;"
                        label-update-restart="&updateRestart.panelUI.label2;"
                        oncommand="PanelUI._onBannerItemSelected(event)"
@@ -632,22 +635,22 @@
                        class="subviewbutton subviewbutton-nav"
                        label="&moreMenu.label;"
                        closemenu="none"
                        oncommand="PanelUI.showSubView('appMenu-moreView', this)"/>
         <toolbarbutton id="appMenu-developer-button"
                        class="subviewbutton subviewbutton-nav"
                        label="&webDeveloperMenu.label;"
                        closemenu="none"
-                       oncommand="PanelUI.showSubView('PanelUI-developer', this, null, true)"/>
+                       oncommand="PanelUI.showSubView('PanelUI-developer', this)"/>
         <toolbarbutton id="appMenu-help-button"
                        class="subviewbutton subviewbutton-iconic subviewbutton-nav"
                        label="&appMenuHelp.label;"
                        closemenu="none"
-                       oncommand="PanelUI.showSubView('PanelUI-helpView', this, null, true)"/>
+                       oncommand="PanelUI.showSubView('PanelUI-helpView', this)"/>
 #ifndef XP_MACOSX
         <toolbarseparator/>
         <toolbarbutton id="appMenu-quit-button"
                        class="subviewbutton subviewbutton-iconic"
 #ifdef XP_WIN
                        label="&quitApplicationCmdWin2.label;"
                        tooltiptext="&quitApplicationCmdWin2.tooltip;"
 #else
@@ -659,17 +662,17 @@
       </vbox>
     </panelview>
     <panelview id="appMenu-moreView" title="&moreMenu.label;" class="PanelUI-subView">
       <vbox class="panel-subview-body">
         <toolbarbutton id="appMenu-characterencoding-button"
                        class="subviewbutton subviewbutton-nav"
                        label="&charsetMenu2.label;"
                        closemenu="none"
-                       oncommand="PanelUI.showSubView('PanelUI-characterEncodingView', this, null, true)"/>
+                       oncommand="PanelUI.showSubView('PanelUI-characterEncodingView', this)"/>
         <toolbarbutton id="appMenu-workoffline-button"
                        class="subviewbutton"
                        label="&goOfflineCmd.label;"
                        type="checkbox"
                        observes="workOfflineMenuitemState"
                        oncommand="BrowserOffline.toggleOfflineStatus();"/>
       </vbox>
     </panelview>

--- a/browser/components/customizableui/content/panelUI.js
+++ b/browser/components/customizableui/content/panelUI.js
@@ -439,32 +439,32 @@ const PanelUI = {
 
   /**
    * Shows a subview in the panel with a given ID.
    *
    * @param aViewId the ID of the subview to show.
    * @param aAnchor the element that spawned the subview.
    * @param aPlacementArea the CustomizableUI area that aAnchor is in.
    */
-  async showSubView(aViewId, aAnchor, aPlacementArea, aAdopted = false) {
+  async showSubView(aViewId, aAnchor, aPlacementArea) {
     this._ensureEventListenersAdded();
     let viewNode = document.getElementById(aViewId);
     if (!viewNode) {
       Cu.reportError("Could not show panel subview with id: " + aViewId);
       return;
     }
 
     if (!aAnchor) {
       Cu.reportError("Expected an anchor when opening subview with id: " + aViewId);
       return;
     }
 
     let container = aAnchor.closest("panelmultiview,photonpanelmultiview");
     if (container) {
-      container.showSubView(aViewId, aAnchor, null, aAdopted);
+      container.showSubView(aViewId, aAnchor);
     } else if (!aAnchor.open) {
       aAnchor.open = true;
 
       let tempPanel = document.createElement("panel");
       tempPanel.setAttribute("type", "arrow");
       tempPanel.setAttribute("id", "customizationui-widget-panel");
       tempPanel.setAttribute("class", "cui-widget-panel");
       tempPanel.setAttribute("viewId", aViewId);
@@ -475,19 +475,24 @@ const PanelUI = {
         tempPanel.setAttribute("animate", "false");
       }
       tempPanel.setAttribute("context", "");
       document.getElementById(CustomizableUI.AREA_NAVBAR).appendChild(tempPanel);
       // If the view has a footer, set a convenience class on the panel.
       tempPanel.classList.toggle("cui-widget-panelWithFooter",
                                  viewNode.querySelector(".panel-subview-footer"));
 
-      let multiView = document.createElement("panelmultiview");
+      let multiView = document.createElement(gPhotonStructure ? "photonpanelmultiview" : "panelmultiview");
       multiView.setAttribute("id", "customizationui-widget-multiview");
       multiView.setAttribute("nosubviews", "true");
+      multiView.setAttribute("viewCacheId", "appMenu-viewCache");
+      if (gPhotonStructure) {
+        multiView.setAttribute("mainViewId", viewNode.id);
+        multiView.appendChild(viewNode);
+      }
       tempPanel.appendChild(multiView);
       multiView.setAttribute("mainViewIsSubView", "true");
       multiView.setMainView(viewNode);
       viewNode.classList.add("cui-widget-panelview");
 
       let viewShown = false;
       let panelRemover = () => {
         viewNode.classList.remove("cui-widget-panelview");
@@ -495,17 +500,19 @@ const PanelUI = {
           CustomizableUI.removePanelCloseListeners(tempPanel);
           tempPanel.removeEventListener("popuphidden", panelRemover);
 
           let evt = new CustomEvent("ViewHiding", {detail: viewNode});
           viewNode.dispatchEvent(evt);
         }
         aAnchor.open = false;
 
-        this.multiView.appendChild(viewNode);
+        // Ensure we run the destructor:
+        multiView.instance.destructor();
+
         tempPanel.remove();
       };
 
       // Emit the ViewShowing event so that the widget definition has a chance
       // to lazily populate the subview with things.
       let detail = {
         blockers: new Set(),
         addBlocker(aPromise) {