--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -1188,29 +1188,53 @@ function RedirectLoad({ target: browser,
       }
     };
     Services.obs.addObserver(delayedStartupFinished,
                              "browser-delayed-startup-finished");
   }
 }
 
 if (document.documentElement.getAttribute("windowtype") == "navigator:browser") {
-  addEventListener("DOMContentLoaded", function() {
+  document.addEventListener("MozBeforeInitialXULLayout", function() {
+    gBrowserInit.onBeforeInitialXULLayout();
+  }, { once: true });
+  document.addEventListener("DOMContentLoaded", function() {
     gBrowserInit.onDOMContentLoaded();
   }, { once: true });
 }
 
 let _resolveDelayedStartup;
 var delayedStartupPromise = new Promise(resolve => {
   _resolveDelayedStartup = resolve;
 });
 
 var gBrowserInit = {
   delayedStartupFinished: false,
 
+  onBeforeInitialXULLayout() {
+    // Set a sane starting width/height for all resolutions on new profiles.
+    if (Services.prefs.getBoolPref("privacy.resistFingerprinting")) {
+      // When the fingerprinting resistance is enabled, making sure that we don't
+      // have a maximum window to interfere with generating rounded window dimensions.
+      document.documentElement.setAttribute("sizemode", "normal");
+    } else if (!document.documentElement.hasAttribute("width")) {
+      const TARGET_WIDTH = 1280;
+      const TARGET_HEIGHT = 1040;
+      let width = Math.min(screen.availWidth * .9, TARGET_WIDTH);
+      let height = Math.min(screen.availHeight * .9, TARGET_HEIGHT);
+
+      document.documentElement.setAttribute("width", width);
+      document.documentElement.setAttribute("height", height);
+
+      if (width < TARGET_WIDTH && height < TARGET_HEIGHT) {
+        document.documentElement.setAttribute("sizemode", "maximized");
+      }
+    }
+  },
+
   onDOMContentLoaded() {
     window.QueryInterface(Ci.nsIInterfaceRequestor)
           .getInterface(nsIWebNavigation)
           .QueryInterface(Ci.nsIDocShellTreeItem).treeOwner
           .QueryInterface(Ci.nsIInterfaceRequestor)
           .getInterface(Ci.nsIXULWindow)
           .XULBrowserWindow = window.XULBrowserWindow;
     window.QueryInterface(Ci.nsIDOMChromeWindow).browserDOMWindow =
@@ -1240,35 +1264,16 @@ var gBrowserInit = {
       if (linkedBrowser) {
         remoteType = linkedBrowser.remoteType;
         isRemote = remoteType != E10SUtils.NOT_REMOTE;
         sameProcessAsFrameLoader = linkedBrowser.frameLoader;
       }
       initBrowser.removeAttribute("blank");
     }
 
-    // Set a sane starting width/height for all resolutions on new profiles.
-    if (Services.prefs.getBoolPref("privacy.resistFingerprinting")) {
-      // When the fingerprinting resistance is enabled, making sure that we don't
-      // have a maximum window to interfere with generating rounded window dimensions.
-      document.documentElement.setAttribute("sizemode", "normal");
-    } else if (!document.documentElement.hasAttribute("width")) {
-      const TARGET_WIDTH = 1280;
-      const TARGET_HEIGHT = 1040;
-      let width = Math.min(screen.availWidth * .9, TARGET_WIDTH);
-      let height = Math.min(screen.availHeight * .9, TARGET_HEIGHT);
-
-      document.documentElement.setAttribute("width", width);
-      document.documentElement.setAttribute("height", height);
-
-      if (width < TARGET_WIDTH && height < TARGET_HEIGHT) {
-        document.documentElement.setAttribute("sizemode", "maximized");
-      }
-    }
-
     gBrowser.updateBrowserRemoteness(initBrowser, isRemote, {
       remoteType, sameProcessAsFrameLoader
     });
 
     gUIDensity.init();
 
     if (AppConstants.CAN_DRAW_IN_TITLEBAR) {
       gDragSpaceObserver.init();

--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -2657,38 +2657,39 @@ XULDocument::DoneWalking()
         // the |if (!mDocumentLoaded)| check above and since
         // mInitialLayoutComplete will be false will follow the else branch
         // there too.  See the big comment there for how such reentry can
         // happen.
         mDocumentLoaded = true;
 
         NotifyPossibleTitleChange(false);
 
+        nsContentUtils::DispatchTrustedEvent(
+            this,
+            static_cast<nsIDocument*>(this),
+            NS_LITERAL_STRING("MozBeforeInitialXULLayout"),
+            true,
+            false);
+
         // Before starting layout, check whether we're a toplevel chrome
         // window.  If we are, setup some state so that we don't have to restyle
         // the whole tree after StartLayout.
         if (nsCOMPtr<nsIDocShellTreeItem> item = GetDocShell()) {
             nsCOMPtr<nsIDocShellTreeOwner> owner;
             item->GetTreeOwner(getter_AddRefs(owner));
             if (nsCOMPtr<nsIXULWindow> xulWin = do_GetInterface(owner)) {
                 nsCOMPtr<nsIDocShell> xulWinShell;
                 xulWin->GetDocShell(getter_AddRefs(xulWinShell));
                 if (SameCOMIdentity(xulWinShell, item)) {
                     // We're the chrome document!
                     xulWin->BeforeStartLayout();
                 }
             }
         }
 
-        nsContentUtils::DispatchTrustedEvent(this,
-                static_cast<nsIDocument*>(this),
-                NS_LITERAL_STRING("MozBeforeInitialXULLayout"),
-                true,
-                false);
-
         StartLayout();
 
         if (mIsWritingFastLoad && IsChromeURI(mDocumentURI))
             nsXULPrototypeCache::GetInstance()->WritePrototype(mMasterPrototype);
 
         NS_ASSERTION(mDelayFrameLoaderInitialization,
                      "mDelayFrameLoaderInitialization should be true!");
         mDelayFrameLoaderInitialization = false;