Fix https://github.com/uBlockOrigin/uBlock-issues/issues/2668

Use document.append() only when there are no elements in
the DOM, as a document instance can't have more than one
direct child.

platform/firefox/vapi-background-ext.js

@@ -315,7 +315,7 @@ vAPI.scriptletsInjector = ((doc, details) => {
         script = doc.createElement('script');
         script.async = false;
         script.src = url;
-        doc.append(script);
+        (doc.head || doc.documentElement || doc).append(script);
         self.uBO_scriptletsInjected = details.filters;
     } catch (ex) {
     }