A must have! Rated 5 out of 5 stars
On every new version of Firefox (3.0, 3.5), I deactivate it (or it isn't available yet), to see if tab management has improved.
And each time, I quickly install it back (thanks, SonjaElen, for the link!)...
Closing a tab with its close icon even if not active (and choice of alternative methods, like Ctrl+Click). Opening a new tab next to the current one. Having tabs on 2 or more rows instead of this scrolling stuff. Being able to unclose a tab (the latest one or one of the MRU). Nicer (IMHO) symbol for moving tabs. The list is endless... (almost).
A must have, IMO.