For me it was split tabs, and vertical nested tabgroups. But I switched over to Zen-Browser by now as Arc on windows is atrocious and it isn’t even available on linux.

I am currently use Outline. Pretty nice and stable. First setup isn’t that easy sadly.

Already switched to Zen. It’s great! Much better community, less buggy, and the dev actually knows what they’re doing. Split tabs still need improvement tho.

Just switched to it and really liking it so far. But it’s missing folders for now… But they say they will push them the day Mozilla releases them to stable Firefox. I hope this project survives!