My favorite feature about Firefox is the fact that it remembers passwords allot better than IE, and if you, for whatever reason, lose your browser (if it crashes on you) you can restore your session.

Also like mentioned above, you can get awesome SEO add ons and many many other helpful plugins that IE doesn't offer.

Only thing I don't like about it, is if you are building a website, you'll have to create 2 different style sheets in order for the site to appear the same in both browsers (IE FF).

This is just my experience.