This fork is obsolete, Panorama View has been updated. Also another fork with more features called Panorama Tab Groups.

This was the best one for a bit, but seemed to get buggier lately. For instance, whenever Firefox loses focus, all my tabs would come back. Activating panorama would restore the groups, but pretty annoying. I've had better luck with https://addons.mozilla.org/en-US/firefox/addon/panorama-tab-groups/ and it seems to be much more actively maintained.

The best one so far for panoramic view of all your opened tabs , but on my test profile it failed with groups. I made 3 groups: Main(93 tabs), Test2(78 tabs), test1(38 tabs ). After restarting Firefox I lost all the groups, and all the tabs merged together into one group named (group0) with 209 tabs. I think this bug exist because I expanded the tab group handle way down till I had to scroll and that is for the group with 93 tabs. Why do I say that? Because every time I expand the handle way down till I get the scroll bar I lose the group and the group names. that per my testing.

PS: Currently, I would suggest using this add-on with one group and just use the panorama view feature which makes life easy. for me having groups is important, so I will wait. And, if your group have many tabs, just zoom out and you will see small thumbnails; it is not an intended feature, but it is helpful.

update: it is too buggy to be used even for just panorama view of your tabs because when moving a tab to another window the tab get a "hide" value AKA it becomes hidden. and sometimes it happens even when you are not moving tabs from a window to another.... I hope you work on this <3 :(