Does Firefox do reproducible builds? This bug report makes me think it doesn’t (at least for Linux): https://bugzilla.mozilla.org/show_bug.cgi?id=885777

But maybe they do for Windows/Apple/Android?

https://qutebrowser.org/ and Librewolf

You can, but the last time I did that the instructions were incomplete, and you still had to auth through Mozilla. That was a few years ago, so things might have changed since then.

If you liked vimperator, you might like https://qutebrowser.org/