FF bug workaround enforce style order at window.onload

see #461

content/apply.js

@@ -517,10 +517,20 @@
       if (!prevExpected) {
         return;
       }
-      if (!CHROME && !force && window !== top) {
-        requestAnimationFrame(() => sortStyleElements({force: true}));
+      if (!CHROME && !force && document.readyState === 'loading') {
+        /*
+          FF bug workaround, see https://github.com/openstyles/stylus/issues/461
+
+          First we move the styles after HEAD because some libraries like Modernizr
+          may add a temporary style element into BODY/HTML and assume it's the last styleSheet
+          consequently failing to access its cssRules as Firefox P R O T E C C S
+          style elements added by an extension from being accessed in a page script.
+
+          Then we wait for the page to load completely, and rearrange the styles again.
+        */
+        addEventListener('load', () => sortStyleElements({force: true}), {once: true});
+        prevExpected = {nextElementSibling: document.head};
         scheduledSort = true;
-        return;
       }
       for (const el of styleElements.values()) {
         if (!isMovable(el)) {