Another nice feature of Tree Style Tab is that it allows you take advantage of the recent trend of "wide screen" displays on laptop screens and desktop monitors. By moving the tab bar to the sides, this extension reclaims vertical space on your screen and replaces it with horizontal space which is more plentiful on most people's screens these days. This is especially true with netbooks which usually have 1024x600 screens. Finally, Tree Style Tab can be set to automatically shrink or even hide the tab bar which reclaims even more space on your screen.
Overall, Tree Style Tab is one of the best Firefox add-ons I've seen.
Update: After a couple days using Tree Style Tab, I've found that while it's a bit rough around the edges, it fits very nicely into my browsing style and really complements the way I use Firefox. For example, I use Google Reader to read blogs and it has a rather convenient "Popout" feature which allows me to open embedded videos in their own window. While this is a nice feature, Tree Style Tab can also force this new window to open as a tab which expands to the size of my existing browser window. This is good as it allows almost full screen video for older computers with flaky video drivers which sometimes gag on full-screen video. (I have an older laptop which resembles that remark. Tree Style Tab can also this for almost any kind of pop up window and I tend to find pop up tabs preferable to pop up windows.