You should check out Tree Style Tab [1]. I always have a lot of tabs open and I can't live without it. It lets you choose where to put the tab bar, among other settings.
It would be better to choose a simpler add-on, such as the ones recommended by the Tree Style Tab developer at http://piro.sakura.ne.jp/xul/_treestyletab.html.en, in the section “Similar or Related Extensions”.
> I want a simple vertical tab, without tree features.
Tree Style Tab has lots of bugs for me, like permanently hiding the navigation bar after I exit full-screen, and scrolling all the way to the bottom of the tab list whenever I restore a closed tab. I still use it because I love being able to organize my tabs hierarchically, but if someone only wants vertical tabs, they should choose a simpler add-on that is less likely to have bugs.
