Hmm, solving a problem that isn't really there?
Whilst it looks nice and seems to work well, I don't actually see it as a problem that needs solving.
If I have too many tabs open in FireFox and want to group similar ones together, I just drag one off to the task bar which pops it into a new window on it's own, and then drag the similar ones to that window. Different tabs in different groups (windows). Done.