Firefox, has very useful features with great customization. IE tab for those sites that won't work in Firefox.
I love Firefox's tab features, if you accidentally close a tab you can restore it, up to the start of your session. You can even automatically open the same tabs you had again when you open Firefox. Even more, it remembers the history of every tab even if you closed it!
With add-ons as add-block plus, IE-tab, Firebug (I just love it), spell-check in multiple languages (build in functionality), FasterFox (to make it faster than most other browsers, otherwise it's rather slow), Greasemonkey, Stylish (to edit your favourite websites to suit your needs) and fully customizable menu bars it's almost impossible for any browser to beat it for me.
There's only one huge disadvantage of Firefox... it uses a lot of system resources.
Javascript in Firefox eats up CPU (well I got a very fast computer so hardly notice it, but when other apps are running...), long sessions eat up to 1GB of ram and it isn't the fastest in starting and closing either (when you look at the task manager, you can see it takes Firefox very long to close).
If I were to make a list:
Firefox > Opera > Konqueror > Safari > I.E. 7
If I would be stuck using I.E. 6 I wouldn't even surf the web, except for some important things. Worst browser ever.