Honestly, I can’t remember specifics but I read some bad stuff about Librewolf a few months ago (nothing nefarious, just seems the developers didn’t necessarily really know what they were doing and made some weird design choices).

I trust the Tor project somewhat, so I tend to trust Mullvad browser more.

Edit: here’s some good information: https://github.com/mullvad/mullvad-browser/issues/1