I think the best vertical tabs implementation in firefox is Sidebery. The use of "panes" to group tabs is brilliant. Older versions were buggy, but version 5 has been rock solid for me.
I am excited that FireFox is working this in by default so I don't have to keep fiddling with userChrome.css to get rid of the top tab bar.
Hey, look on the bright side: maybe chromium will get vertical tabs soon!
Chrome is faster, snappier and works better on more websites I commonly use, but the fact that I cannot have "vertical tabs as trees" ruins the entire browser experience for me, so it's basically the only reason I use Firefox for the last decade or something.
[1] https://github.com/gorhill/uBlock/wiki/Blocking-mode:-medium...
I kinda dislike that Firefox only have one good option that involves completely hiding each group currently not in use, but it functioned ith their tab containers which made it worth the hassle.
If this does too, I'm switching permanently
Sidebery is nice, but it's missing an API allowing other addons to interact with it. This is a big benefit of Tree Style Tabs, especially as you can even exploit it as a user.
Their wiki also has a very simple and effective userChrome.css tweak to hide the top tab bar when the side panel is open. That's a rather crucial vertical space savings on a small laptop.