Tree style tabs is a vertical tab extension for ff and the best ui around if you use a lot of tabs. I can't even use chromium vertical tabs anymore because they feel terrible in comparison. You can also fully style tree style tabs via css.
But then you get duplicates, since Mozilla made it a pain to disable the main tab bar. I'm using a Chromium offshoot (Arc) but I also like how Vivaldi handles vertical tabs. Firefox doesn't just compete with Chromium itself, but the entire ecosystem.
Does Vivaldi nest tabs yet? Its been a few years since I tried it but it didn't use to.
With TST, I can open a link as a new tab (middle click) and it is shown as a child of the opening tab. This makes it easy to minimise or close a whole tree of tabs.
you can stack tabs in a group, and when you're inside that group, new tabs are opened within that group, but not sure about the middle click for a single tab
It also has a great alternative - workspaces, that helped me much better vs. the stacking, that for some reason didn't stick
https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...