I found a good guide for firefox tuning online, what I missed was a simple way to apply the changes described there in the way of doing it using about:config of firefox and doing everything in the gui manually. It’s easier. Change into your firefox user directory for windows this should be something like (sorry for the german in the path 😉 :
C:\Dokumente und Einstellungen\Administrator\Anwendungsdaten\Mozilla\Firefox\Profiles\<Your_Profile_Name>\prefs.js
put the following lines somewhere in bettween there:
save the file, restart firefox and have a look.