--- a/browser/components/extensions/ext-browserAction.js
+++ b/browser/components/extensions/ext-browserAction.js
@@ -18,18 +18,16 @@ XPCOMUtils.defineLazyModuleGetter(this, 
                                   "resource://gre/modules/TelemetryStopwatch.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "ViewPopup",
                                   "resource:///modules/ExtensionPopups.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(this, "DOMUtils",
                                    "@mozilla.org/inspector/dom-utils;1",
                                    "inIDOMUtils");
 
-XPCOMUtils.defineLazyPreferenceGetter(this, "gPhotonStructure", "browser.photon.structure.enabled");
-
 var {
   DefaultWeakMap,
 } = ExtensionUtils;
 
 Cu.import("resource://gre/modules/ExtensionParent.jsm");
 
 var {
   IconDetails,
@@ -51,17 +49,17 @@ const isAncestorOrSelf = (target, node) 
 };
 
 // WeakMap[Extension -> BrowserAction]
 const browserActionMap = new WeakMap();
 
 XPCOMUtils.defineLazyGetter(this, "browserAreas", () => {
   return {
     "navbar": CustomizableUI.AREA_NAVBAR,
-    "menupanel": gPhotonStructure ? CustomizableUI.AREA_FIXED_OVERFLOW_PANEL : CustomizableUI.AREA_PANEL,
+    "menupanel": CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
     "tabstrip": CustomizableUI.AREA_TABSTRIP,
     "personaltoolbar": CustomizableUI.AREA_BOOKMARKS,
   };
 });
 
 this.browserAction = class extends ExtensionAPI {
   static for(extension) {
     return browserActionMap.get(extension);
@@ -241,21 +239,17 @@ this.browserAction = class extends Exten
       return;
     }
 
     // Popups are shown only if a popup URL is defined; otherwise
     // a "click" event is dispatched. This is done for compatibility with the
     // Google Chrome onClicked extension API.
     if (this.getProperty(tab, "popup")) {
       if (this.widget.areaType == CustomizableUI.TYPE_MENU_PANEL) {
-        if (gPhotonStructure) {
-          await window.document.getElementById("nav-bar").overflowable.show();
-        } else {
-          await window.PanelUI.show();
-        }
+        await window.document.getElementById("nav-bar").overflowable.show();
       }
 
       let event = new window.CustomEvent("command", {bubbles: true, cancelable: true});
       widget.node.dispatchEvent(event);
     } else {
       this.emit("click");
     }
   }

--- a/browser/components/extensions/test/browser/browser_ext_browserAction_pageAction_icon.js
+++ b/browser/components/extensions/test/browser/browser_ext_browserAction_pageAction_icon.js
@@ -288,38 +288,12 @@ add_task(async function testDetailsObjec
       is(isLegacy, test.legacy, "Legacy class should be present?");
 
       await SpecialPowers.popPrefEnv();
     }
 
     if (!test.menuResolutions) {
       continue;
     }
-
-
-    if (!gPhotonStructure) {
-      // Test icon sizes in the menu panel.
-      CustomizableUI.addWidgetToArea(browserActionWidget.id,
-                                     CustomizableUI.AREA_PANEL);
-
-      await showBrowserAction(extension);
-      browserActionButton = browserActionWidget.forWindow(window).node;
-
-      for (let resolution of Object.keys(test.menuResolutions)) {
-        await SpecialPowers.pushPrefEnv({set: [[RESOLUTION_PREF, resolution]]});
-
-        is(window.devicePixelRatio, +resolution, "window has the required resolution");
-
-        let imageURL = test.menuResolutions[resolution];
-        is(getListStyleImage(browserActionButton), imageURL, `browser action has the correct menu image at ${resolution}x resolution`);
-
-        await SpecialPowers.popPrefEnv();
-      }
-
-      await closeBrowserAction(extension);
-
-      CustomizableUI.addWidgetToArea(browserActionWidget.id,
-                                     CustomizableUI.AREA_NAVBAR);
-    }
   }
 
   await extension.unload();
 });

--- a/browser/components/extensions/test/browser/browser_ext_browserAction_popup_resize.js
+++ b/browser/components/extensions/test/browser/browser_ext_browserAction_popup_resize.js
@@ -153,17 +153,17 @@ async function testPopupSize(standardsMo
 
 
   // Test the PanelUI panel for a menu panel button.
   let widget = getBrowserActionWidget(extension);
   CustomizableUI.addWidgetToArea(widget.id, getCustomizableUIPanelID());
 
   let browser = await openPanel(extension, browserWin);
 
-  let panel = gPhotonStructure ? browserWin.PanelUI.overflowPanel : browserWin.PanelUI.panel;
+  let panel = browserWin.PanelUI.overflowPanel;
   let origPanelRect = panel.getBoundingClientRect();
 
   // Check that the panel is still positioned as expected.
   let checkPanelPosition = () => {
     is(panel.getAttribute("side"), arrowSide, "Panel arrow is positioned as expected");
 
     let panelRect = panel.getBoundingClientRect();
     if (arrowSide == "top") {

--- a/browser/components/extensions/test/browser/browser_ext_popup_corners.js
+++ b/browser/components/extensions/test/browser/browser_ext_popup_corners.js
@@ -1,15 +1,13 @@
 /* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
 /* vim: set sts=2 sw=2 et tw=80: */
 "use strict";
 
 add_task(async function testPopupBorderRadius() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
-
   let extension = ExtensionTestUtils.loadExtension({
     background() {
       browser.tabs.query({active: true, currentWindow: true}, tabs => {
         browser.pageAction.show(tabs[0].id);
       });
     },
 
     manifest: {

--- a/browser/components/extensions/test/browser/browser_ext_themes_icons.js
+++ b/browser/components/extensions/test/browser/browser_ext_themes_icons.js
@@ -165,30 +165,16 @@ async function runTestWithIcons(icons) {
   }
 
   let extension = ExtensionTestUtils.loadExtension({manifest, files});
 
   await extension.startup();
 
   checkButtons(icons, ICON_INFO, "toolbar");
 
-  if (!gPhotonStructure) {
-    for (let button of ICON_INFO) {
-      if (button[2]) {
-        CustomizableUI.addWidgetToArea(button[2], CustomizableUI.AREA_PANEL);
-      }
-    }
-
-    await PanelUI.show();
-
-    checkButtons(icons, ICON_INFO, "panel");
-
-    await PanelUI.hide();
-  }
-
   await extension.unload();
 
   for (let button of ICON_INFO) {
     verifyButtonWithoutCustomStyling(button[1],
       `The ${button[1]} should not have its icon customized when the theme is unloaded`);
   }
 }
 

--- a/browser/components/extensions/test/browser/head.js
+++ b/browser/components/extensions/test/browser/head.js
@@ -209,48 +209,40 @@ var awaitExtensionPanel = async function
 
     awaitLoad && awaitBrowserLoaded(browser),
   ]);
 
   return browser;
 };
 
 function getCustomizableUIPanelID() {
-  return gPhotonStructure ? CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
-                          : CustomizableUI.AREA_PANEL;
+  return CustomizableUI.AREA_FIXED_OVERFLOW_PANEL;
 }
 
 function getBrowserActionWidget(extension) {
   return CustomizableUI.getWidget(makeWidgetId(extension.id) + "-browser-action");
 }
 
 function getBrowserActionPopup(extension, win = window) {
   let group = getBrowserActionWidget(extension);
 
   if (group.areaType == CustomizableUI.TYPE_TOOLBAR) {
     return win.document.getElementById("customizationui-widget-panel");
   }
-  return gPhotonStructure ? win.PanelUI.overflowPanel : win.PanelUI.panel;
+  return win.PanelUI.overflowPanel;
 }
 
 var showBrowserAction = async function(extension, win = window) {
   let group = getBrowserActionWidget(extension);
   let widget = group.forWindow(win);
 
   if (group.areaType == CustomizableUI.TYPE_TOOLBAR) {
     ok(!widget.overflowed, "Expect widget not to be overflowed");
   } else if (group.areaType == CustomizableUI.TYPE_MENU_PANEL) {
-    // Show the right panel. After Photon is turned on permanently, this
-    // can be re-simplified. This is unfortunately easier than getting
-    // and using the panel (area) ID out of CustomizableUI for the widget.
-    if (gPhotonStructure) {
-      await win.document.getElementById("nav-bar").overflowable.show();
-    } else {
-      await win.PanelUI.show();
-    }
+    await win.document.getElementById("nav-bar").overflowable.show();
   }
 };
 
 var clickBrowserAction = async function(extension, win = window) {
   await promiseAnimationFrame(win);
   await showBrowserAction(extension, win);
 
   let widget = getBrowserActionWidget(extension).forWindow(win);

--- a/browser/themes/shared/customizableui/panelUI.inc.css
+++ b/browser/themes/shared/customizableui/panelUI.inc.css
@@ -333,17 +333,16 @@ panelview:not([mainview]) .toolbarbutton
   padding: 4px 0;
 }
 
 /* START photonpanelview adjustments */
 
 #appMenu-popup > .panel-arrowcontainer > .panel-arrowcontent,
 panel[photon] > .panel-arrowcontainer > .panel-arrowcontent {
   padding: 0;
-  border-radius: 0;
 }
 
 photonpanelmultiview panelview {
   background: var(--arrowpanel-background);
   padding: 0;
 }
 
 #appMenu-popup panelview,