If you like to have different Firefox add-ons, settings and themes, depending on whether you're surfing for fun, working, blogging etc. then this screencast will get you set up with different profiles. You can also use this method of running Firefox if you like trying the latest alpha and beta versions of Firefox, but you don't want to get your add-ons all messed up.
On Mac, start Firefox with this command in terminal:
/Applications/Firefox.app/Contents/MacOS/firefox-bin --profilemanagerTomorrow,
I'll show you how to do this in Windows, but if you think you can do it on your own, here's the command you'll need to paste into the run window (Start->run):
"C:\Program Files\Mozilla Firefox\firefox.exe" --profilemanager