Great news, I use tab groups daily in Safari and recently in Firefox beta.
In Firefox, tab groups work better with a vertical tab bar.
In Safari, there is a feature that I wish comes to Firefox: that the tabs in each group act as bookmarks, i.e., they persist across browser restarts and new windows. They are always there within their group; only the tabs that are not in a group are volatile.
This makes tab groups more useful, as you don't fear losing them no matter what.
Firefox tab groups persist across browser restarts (or at least mine have for ~5 months now). I haven't had any issues with new windows yet either though I have definitely lost Firefox tabs to that issue in the past.
I agree though, Chrome does a similar implementation where tab groups act like a special class of bookmarks and it's much nicer. You have the same process where you never have to fear losing groups and you can 'hide' tab groups that you aren't actively using and they're potentially accessible from any window.
In Firefox, tab groups work better with a vertical tab bar.
In Safari, there is a feature that I wish comes to Firefox: that the tabs in each group act as bookmarks, i.e., they persist across browser restarts and new windows. They are always there within their group; only the tabs that are not in a group are volatile.
This makes tab groups more useful, as you don't fear losing them no matter what.