A great power of Firefox are its add-ons, aren't they? Sidebery [1] has been a solid implementation of vertical tabs since a long time ago. Begore that got popular, Tree Style Tabs [2] was also a very comprehensive solution.

But nowadays, vertical tabs are native since Firefox v136 [3][4], so at least for the basics you won't need an add-on.

[1]: https://addons.mozilla.org/firefox/addon/sidebery/

[2]: https://addons.mozilla.org/firefox/addon/tree-style-tab/

[3]: https://www.hackerneue.com/item?id=41192118

[4]: https://www.hackerneue.com/item?id=43254871