Tab groups with colors is great but they cause clutter. Once you have enough, they're no different from tabs themselves; just complete chaos at the top of the screen. Keep them as a neat feature but introduce Sessions. Opera already does this. Its like multiple instances of the same browser running inside the same window. This would solve the clutter and allow the users to actually use your tab grouping even more efficiently.
I really like Firefox and been using it as my main browser for about a year now but it handles some quality of life stuff poorly. You can make it a lot better.