I love this extension, it's usually the first one I install, & should be incorporated as a standard feature of ff ( along with vertical tabs in the sidebar & per-page RAM deallocation ). Its only complication is that ( on slower machines ) it can't execute before a loading displays its white background ( caused by ff not this ext. ). ( Extensively tried in Linux & Windows10 ).

And it doesn't pilfer my private data. Viva la libre!