[Updated review]*
There are a number of glitches with this that can still be worked out, but some of them can sort of be considered features in a way. Not too sure how much support for this Firefox has, but core issues that previously existed (a key one being that closing a window with a tab group that was not the last window opened disabled the function of tab groups that were opened, meaning tabs needed to be reassigned into a group to function again -- this is no longer the case as they groups function when windows are re-opened using History)

Groups Tabs do not have the ability to collapse the group they belong to still, so the workaround is to have a tab outside of the group to click on to collapse the groups more easily

Moving multiple tabs into a Group Tab will move each tab individually - which can be a slow process. If you are mass moving tabs into a Group, please make sure you do not change the Group Tab name in the middle of the process of the tabs being moved, or you may end up "losing" some tabs (they exist, they're just not accessible until you fiddle around a bit such as disable the Extension due to them being hidden under a group that exists but isn't visible on your tabs list)

Moving multiple tabs within a group to change their position can also glitch up a bit. Normally, if you move tabs individually, they will be confined to the bounds of the Group. If you move them as a group, this can potentially glitch up and allow tabs to be positioned outside of the bounds of the Group (eg. very front of the entire set of tabs in the window and in front of all sorts of other tabs that are in front of the Group Tab -- instead of being at the very front of the set of tabs contained within the Group and immediately behind the Group Tab). I consider this almost a feature. Issue is... this messes up the order of tab positions, so your new tabs may end up popping up in unexpected places and not immediately behind the tab you are opening said new tab from

Overall, fairly usable with a few glitches here and there. If you've tried this extension before and had issues with it (eg. the whole issue of Group Tabs not functioning after reopening a closed window during a session), it may be worth checking out once more to see if the issues you've had were resolved

This is where it's at, if you need tab groups for Firefox. The other extensions are too odd and obtuse for me, and this replicates Chrome-like grouping.

Closest tab grouping I've found compared to Edge's "tab islands." Having tabs grouped and hidden but leaving behind a group tab is very helpful. Thank you!

A way to keep tab groups across browsing sessions (after browser restart) is needed.

Colored grouping was mentioned before and would help with visual organization. I'd also take colored/unique tab group favicons to more easily see the difference between my groups.

The ability to keep a tab group expanded when none of the tabs are active would help if/when group coloring exists so that all website titles and favicons can remain visible yet clearly grouped.

Very good extension. Things missing off the top of my head are tabs being grouped by color and a way to backup or keep them consistent across devices.

If you're looking for a direct alternative to the way chrome groups tabs, this is it. I'm not joking, I looked for 3 hours. There are definitely improvements that could be made to the appearance of it all but the pure functionality outweighs that.