FireTune (freeware, Windows version) works really very well. Have a try!

FireTune for Mozilla Firefox v1.x was developed for an easy and fast optimization of your browsing experience with Firefox. Usually you have to optimize Firefox manually, which can be time consuming and difficult for the novice user. FireTune helps you here – it includes all the performance optimizations. The only thing you must do is: make your selection. It does the work for you.