I use Vivaldi because of its Workspaces which have so much better UX than tab groups. I don't need to see my Social or my Tech tab group until I'm switching for it; Firefox shows them all the time - atleast I haven't found a way to hide them and associated tabs until I need them without having to launch a new window. In Vivaldi, they are ready to use immediately upon switching while keeping everything in the same window keeping my taskbar clean. All this while not making my CPU fans run like a jet engine.

I would rather have this experience with Firefox but they are probably more busy focusing on email, vpn and whatever the flavor of the month is.