@mfat It's the old problem about bugs. To know that a piece of software has no bugs you should be able to count them and if you could do it then should be able to locate them and make a fix. But you can't then there's no way to know there's no more undetected backdoor
Of course being open source helps a lot but there's no solver bullet
@KarnaSubarna also found some interesting style snippets for Firefox 😀
https://github.com/mbnuqw/sidebery/wiki/Firefox-Styles-Snippets-(via-userChrome.css)