I've been a long time Firefox user, but the UI/UX has stagnated (like all browsers) for so long. I'm enjoying using Arc at the moment because it's rethinking a lot of things that bug me about the traditional browser UI paradigm but I also know that it's a trendy startup that could either: explode, decide to change features radically at the drop of a hat (pivot), or simple do something distasteful to please investors.
Extensions technically fill in the gap on the Firefox side, like Containers, but the UX is just an afterthought.