It's a combination of FireFox being the least shit and the most functional option, really. Still kind of shit by default, so I go for Librewolf. Also I've been using it since Firebird.
Librewolf just gives me more sane defaults. Saves me trouble of having to dig in to about:config and trying to figure out what needs to be changed. Generally its releases follow FireFox in reasonable time. A few updates have messed things up, but it's rare. I guess the big thing is that Librewolf saves me some effort.