So I started using Mozilla Firefox about a month ago and I think it's official now -- it's my primary browser. It's got so many cool things that IE doesn't have (at least not yet). Things like tabbed browsing, quick searches for links in a page, a much nicer bookmarks/favorites manager, and built-in popup blocking.
The tabbed browsing thing alone is worth the switch, given that I very rarely have only one browser instance open at a time. More often, it's a handful of different articles, google search results, or news stories. I also like that you can save an entire set of tabs as one bookmark and return to them all via that single bookmark.
Another very cool feature is the ability to block images based on the domain in the IMG "src" element. I never see images now that come from the common banner ad servers.
And the extensions/plugins being built are amazing. I use the GoogleBar (essentially the Google toolbar), a Flash blocker (it replaces Flash with a small logo that you can click if you want to view it), and a bunch of web development and related extensions.
Oh yea, it's also free.
So are there things I miss from IE? Sure... there are still a number of sites that don't work well in Firefox. IE's by far the most popular browser, so some designers don't bother to test their sites in other browsers. When I run across those pages, I use a Firefox extension that puts "View this page in IE" on my right-click menu.