--- a/xpfe/appshell/nsWebShellWindow.cpp
+++ b/xpfe/appshell/nsWebShellWindow.cpp
@@ -350,20 +350,16 @@ nsWebShellWindow::SizeModeChanged(nsSize
   if (sizeMode == nsSizeMode_Maximized || sizeMode == nsSizeMode_Fullscreen) {
     uint32_t zLevel;
     GetZLevel(&zLevel);
     if (zLevel > nsIXULWindow::normalZ)
       SetZLevel(nsIXULWindow::normalZ);
   }
   mWindow->SetSizeMode(sizeMode);
 
-  // Persist mode, but not immediately, because in many (all?)
-  // cases this will merge with the similar call in NS_SIZE and
-  // write the attribute values only once.
-  SetPersistenceTimer(PAD_MISC);
   nsCOMPtr<nsPIDOMWindowOuter> ourWindow =
     mDocShell ? mDocShell->GetWindow() : nullptr;
   if (ourWindow) {
     // Ensure that the fullscreen state is synchronized between
     // the widget and the outer window object.
     if (sizeMode == nsSizeMode_Fullscreen) {
       ourWindow->SetFullScreen(true);
     }
@@ -384,16 +380,22 @@ nsWebShellWindow::SizeModeChanged(nsSize
     ourWindow->DispatchCustomEvent(NS_LITERAL_STRING("sizemodechange"));
   }
 
   nsIPresShell* presShell;
   if ((presShell = GetPresShell())) {
     presShell->GetPresContext()->SizeModeChanged(sizeMode);
   }
 
+  // Persist mode immediately as the timer causes race condition
+  // between css rules dependent on window sizemode and
+  // sizemodechange/resize listener which calculates TabsToolbar height.
+  PersistentAttributesDirty(PAD_MISC);
+  SavePersistentAttributes();
+
   // Note the current implementation of SetSizeMode just stores
   // the new state; it doesn't actually resize. So here we store
   // the state and pass the event on to the OS. The day is coming
   // when we'll handle the event here, and the return result will
   // then need to be different.
 }
 
 void