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。

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!