Waterfox is nearly same as Librewolf; it comes with own set of additional settings and tweaks. From what I saw around some users weren't fond of the arrangement

Waterfox is nearly same as Librewolf; it comes with own set of additional settings and tweaks. From what I saw around some users weren't fond of the arrangements the author of the project went into (startpage, fastly) and bundling of tree tabs extension that had to be disabled by about:config entry change. Pale Moon: IIRC initially it was a fork of Firefox ESR then project switched to own Goanna engine, which in turn is a fork of Gecko; it also tries to keep alive XUL extensions platform. There were some dramas happening in the past around the project, esp. regarding porting browser to OpenBSD. It's kinda a niche project I'd say - surely it has some community but it's not that big. As for IceCat - I'm surprised it's still around; seems it runs own extensions database (https://gnuzilla.gnu.org/mozzarella/ https://gnuzilla.gnu.org/mozzarella/) but these aren't keeping up with official releases, at least on the page. Perhaps autoupdate bumps these to the current versions; still, uBlock Origin is stuck at 1.51.0 from last December, while the current version is 1.59.0. I tried Zen recently - it's kinda hard to get used to tabs sidebar. For some people this might be useful but not for me - I prefer tabs on top.