diff --git a/content/base/src/nsDocument.cpp b/content/base/src/nsDocument.cpp
index 41a080f37f1..86a83112739 100644
--- a/content/base/src/nsDocument.cpp
+++ b/content/base/src/nsDocument.cpp
@@ -8907,16 +8907,9 @@ nsDocument::RequestFullScreen(Element* aElement, bool aWasCallerChrome)
   // Remember this is the requesting full-screen document.
   sFullScreenDoc = do_GetWeakReference(static_cast<nsIDocument*>(this));
 
-  // Make the window full-screen. Note we must make the state changes above
-  // before making the window full-screen, as then the document reports as
-  // being in full-screen mode when the chrome "fullscreen" event fires,
-  // enabling chrome to distinguish between browser and dom full-screen
-  // modes. Also note that nsGlobalWindow::SetFullScreen() (which
-  // SetWindowFullScreen() calls) proxies to the root window in its hierarchy,
-  // and does not operate on the a per-nsIDOMWindow basis.
-  SetWindowFullScreen(this, true);
-
 #ifdef DEBUG
+  // Note assertions must run before SetWindowFullScreen() as that does
+  // synchronous event dispatch which can run script which exits full-screen!
   NS_ASSERTION(GetFullScreenElement() == aElement,
                "Full-screen element should be the requested element!");
   NS_ASSERTION(IsFullScreenDoc(), "Should be full-screen doc");
@@ -8926,6 +8919,15 @@ nsDocument::RequestFullScreen(Element* aElement, bool aWasCallerChrome)
   NS_ASSERTION(c->AsElement() == aElement,
     "GetMozFullScreenElement should match GetFullScreenElement()");
 #endif
+
+  // Make the window full-screen. Note we must make the state changes above
+  // before making the window full-screen, as then the document reports as
+  // being in full-screen mode when the chrome "fullscreen" event fires,
+  // enabling chrome to distinguish between browser and dom full-screen
+  // modes. Also note that nsGlobalWindow::SetFullScreen() (which
+  // SetWindowFullScreen() calls) proxies to the root window in its hierarchy,
+  // and does not operate on the a per-nsIDOMWindow basis.
+  SetWindowFullScreen(this, true);
 }
 
 NS_IMETHODIMP