Firefox is an alternative browser to Internet Explorer. The latest updates you see in IE7 are direct results of trying to compete with features that have been in Firefox. Even still, Firefox is faster, more secure, and overall easier to customize (if you are into that). I’ve used Firefox for years now and would never go back to IE.
If you want to have some fun, and help Mozilla (Firefox’s creator) to break a world record, click on the badge to the left. They are trying to break the software download record in a 24-hour period when they release Firefox 3.0.
In the meantime, you can download Firefox 2 from the link above, or if you are a little more adventurous, you can preview Firefox 3 here.