It's not made by Microsoft.
It's free
It's less buggy than Internet Explorer
It uses less memory now the memory leak has been patched.
It has tabbed browsing - I know IE7 has this, but it's so clunky and unfriendly.
It won't automatically run ActiveX controls on sites that you visit unlike IE7 which means that if someone has a nasty little script on their page you're safer with Firefox.
It has literally thousands of free add-ons so you can customise it to your heart's content. For example, I have local weather, an automatic spell checker and a media player built into mine. You can add currency converters, different sounds for actions and different skins to totally change the way it looks.
Want more? ;)