I've been using the Tree Style Tabs Firefox extension for a few months now, and love it! Having vertical tabs has always been fabulous, ever since I first encountered them in Galeon. The chrome-css hack that I have been using for the last few years in Firefox stopped working in Firefox 3 (beta), so I went exploring and am very glad I did.
Not only does the Tree Style Tabs extension give my my vertical tabs, it also gives me (surprise surprise) a tree of them! Each has it's own browsing history, and subtrees can be collapsed and reordered and all that wonderful goo.
Just now I was playing around with the settings and thought I'd try the auto-hide feature. I'm not sure I'll keep it because there is a slight flicker that bothers me... but it is neat none-the-less! The tab bar is hidden, and then when I mouse or keyboard activate it the bar appears translucently on top of the page. It might need some kinks worked out (or perhaps there is some other cause for the flickering), but I think I like it!