Having used them both when forced to do so, I think Firefox is the better of the two options given. There are better add-ins available for it, and it comes with more built-in features. Bugs are usually fixed pretty quickly. It also loads faster. I think that one of the things that put me off IE7 was that I was forced to use a beta version on a shared computer for a period of a few months, which was ridden with several annoying bugs - for example whenever I used the mouse wheel to scroll, it kept on scrolling.
Having said this, I am one of the minority who are still clinging to IE6, because I don't like change. IE6 is becoming harder to use now, and as some websites stop supporting it, I am finding myself using Firefox more and more. I'm actually using it at the moment.