If you're looking for a fun add-on to make some aesthetic customizations to Firefox, you should check out Personas for Firefox. The add-on doesn't really add anything functional to your 'fox installation, but it does add the ability to swap in some pretty nice looking themes.
I didn't think this add-on would be that interesting, but I have to admit that I find Firefox way more pleasant to look at with the themes made available by Personas. You can also swap between themes quickly and easily from a new icon that the add-on places in your status bar.
Thanks to Chris L. for telling me about this!