The Secrets of Firefox about:config
 

While Firefox is very customizable, many of its settings aren't in the Options. Each setting is named and stored as a string, integer, or Boolean in a file called prefs.js and accessed via about:config from the nav bar. Computerworld provides instructions on 20 tweaks for speeding up page loads, making tabs behave, reducing memory drain, and generally making the interface act the way you want it to.

http://slashdot.org/article.pl?sid=0...33256&from=rss

The about:config trick is also available in Opera. :)