I like Firefox & I've been using it for a couple of years now, but I have to admint I don't 'get' the tabbed browsing thing. How is it so much better to have a bunch of tabs open in one window than to just have several windows open?
For me it's a reduction of screen real estate required to have multiple sites open. I can use the keyboard to quickly move from one tab to another without the context switching you get when you go from one window to another.
Also, there are extensions available to make tabs even more flexible. I know folks who want to be able to select a bookmark, and have it open 5 or 6 tabs automatically. I don't use that at the moment, but have been thinking of ways it could help some tasks.
Basically, the UI space saved.