Reasons to Switch

Flexbeta - 13 Reasons To Use Firefox Over IE - Page 1

This site gives a pretty good overview of Firefox's advantages. I'd add that Firefox, like regular Mozilla, allows you to skin the interface (change its appearance - here are some examples) without installing spyware (never install HotBar) or editing the registry. Maybe that's not a particularly useful feature, but it's one of those nifty bells and whistles.
The ability to block ad server images is probably one of my favorite features. If you install the Adblock extension, you can even block Flash ads, too, ads I find almost as annoying as popups.

And tabbed browsing is great, too, because having a whole bunch of IE windows open either crowds your taskbar so that you can't tell the windows apart or, in XP, causes the taskbar to condense all the windows into a popup menu that adds yet another step to getting to your window. Plus it's nice just to be able to control/command-click a link and have it load in a background tab while you finish reading the page you're on, rather than having to right/control-click, choose "Open in new window" and then click back to your original page when the new window blocks the page you were reading.

And maybe my favorite feature is the ability to make bookmark menus on the Favorites/Bookmark Bar. I don't know if IE can do this -- it's something I never knew about until I got my iBook, and I used Mozilla on Windows anyway. It works in IE for Mac, but not as well since the menus aren't sticky (i.e., you have to keep the mouse button down until you reach your selection). But since I have too many bookmarks to fit individually on the Bookmark Bar, I put folders of bookmarks onto the bar, and when I click on them they reveal a bookmark menu. It's like having a second menu bar just for bookmarks. I realize you could always just use the bookmarks menu, but that's not nearly as visible and takes more mousing. It has less room, too.