That’s simply one of the most useful extensions I have ever seen. A perfect tool especially for people who prefer clicking rather than typing, highly customizable and incredibly useful. I’m not going to describe it - the demo movie speaks for itself:

Firefox is safety-oriented. It has plenty of features, which should prevent you from unintentional crashing your browser or system or revealing your private data. They all make sense. However, they can get irritating if you know what you’re doing. If you’re fed up with confirming your choices every 5 minutes, have a look at the possibilities of disabling unnecessary protection mechanisms.

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.