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 :(