I have already described how to create and manage multiple profiles in Firefox. If you haven’t already read it, it may be a useful introduction.
If you have a few Firefox profiles created you will probably find the possibility of running them simultanously really useul. What is more, it enables you installing and using various version of Firefox on one machine - you can easily test and compare Firefox 3 beta to older versions.

I find it really unusual how many people using Firefox for a relatively long time do not know about such useful and basic features like Smart Keywords. Provided with the core of Firefox, they are a great tool for every web surfer. They basically enable you to search web faster and access your favorite websites with your keyboard only.