If you haven't been running one of the release candidates already, you may want to get the latest and perhaps best browser to date, Firefox 1.5, now that it's been officially released. And really, if you're using another browser -- especially the buggy and unsafe Internet Explorer -- you owe it to yourself to at least try Firefox, which is safer for your machine.
That's reason one.
As a designer, Firefox is a pleasurable development in the online world. I can't speak for others, but I think websites look better in Firefox. Meanwhile, Internet Explorer, thanks to Microsoft's defiance of web standards, continues to be a nightmare for web designers who waste additional hours upon hours to hack all the Internet Explorer quirks in CSS so that IE doesn't break the website altogether.