Edit: This hasn't been updated in 10 months, I recommend using Panorama Tab Groups instead. Because of this I've changed the review from 5 to 3 stars.

Great add-on, it's getting very close to feature parity (or at least the ones I need :D) with the old tab groups! I'd tried it out a while back, but it wasn't quite good enough. After realising that resizing had been added and there was a shortcut (that is nearly identical to the old ctrl-shift-e) I was ecstatic.

A QoL things that would be nice to have before I move all my devices to the new firefox (I presume you're working on this):
-Double clicking to create a new tab group, rather than a button on the side
-Being able to move groups by selecting any part of the group (eg. the bottom part of a group)
-Not letting groups overlap, instead rearranging existing groups
-Better representation of when a group can't show all the tabs, only showing the active one is a tad deceiving