I really ask myself why they insist to put the tabs on top. With todays widescreen-displays, putting them left or right makes more sense for most use cases (at least with FF its possible to get this via add-on). OTOH they try to get rid of every pixel to get a bit more space while there are lots of at the left and at the right. Do all developers only work on 13" laptops today?
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.
