Librewolf has been my pick for a couple years now. Some sites break because they can't read the canvas, but the fix is easy - allow the site to access it. I messed up a work contract signature because of this once, and my scribbles looked like random digital noise. It was fun explaining that to finance and having them reissue the contract.

Waterfox took all of like 5 minutes to get up and running and imported all my stuff from Firefox too. Going on maybe two months using it and yet to have any issues. I think you’ll be fine with either choice!

Personally I main Librewolf for a few years now, but you need to remember that cookies are opt-in (shield icon, click the cookie toggle).

Personally I think it's a great feature but some people may prefer cookies always on

Seems either one is good, including Mullvad, depending on your threat model.

I'd say from lowest to highest threat model for Gecko-forked browsers: WaterFox, Librewolf, Mullvad, Tor.