Wow! Easy to use, and does just what it says. Press F1 to show or hide its list. If you wish, it's even possible to hide FF's normal (horizontal) tabs too, though it takes some fussing—see this PCWorld article:
https://www.pcworld.com/article/823939/vertical-tabs-in-firefox-yes-its-really-possible.html
Maybe this extension will eventually add that capability? (If so, they'd want to disable the F1 toggle, because you've got to have tabs somewhere, right?) Thanks for this great idea!

非常好组件，爱来自中国。

整理成树状标签的功能确实很好用，但是将其调整为树状视角时，会出现标签页被被重新打开的情况，这会导致历史记录被重复记录。

Abilty to work

Edit: My last review was an error I made... I found out, that under specific circumstances the data either of Tree Stye Tab (-> TST) or Simple Tab Groups (-> STG) can become corrupt, which makes them behave "strange". I assume it was STG, which caused the issue, because when deleting a few big, but useless groups they shown further strange effects, which vanished after deleting them completely.
Forget my 3-Star-Rating, and my apologies!

There's a rare incompatibility with Simple Tab Groups (->STG).
When saving a session in STG, the tree structure of Tree Style Tab (->TST) get lost completely and as a result the stucture is gone.
This happens on :
Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
OS Linux 6.1.0-7-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.20-2 (2023-04-08)
OS Theme
and is a quite new problem.
Unfortunately I have no real idea how to check, which of both AddOn's produce the "error"...
Does anybody have an idea?
BTW... the rating will be reset after solving the issue, because I know this AddOn since a long time now and used it many times... sometimes in the past though it came to my attention, rhat there were some incompatibilities from time to time - that's why I post this message here and in STG.