Firefox has lots of tweaks/customizations you can make to its behavior which you cannot get to through the normal preferences dialog. To view these options, in your address bar enter: ‘about:config’. When you access the configuration settings, you will be presented with a warning so take care with what you modify.

As you will see when you are scrolling through the options, there will be some entries which make sense as to what they do and many which do not. To help you decode this list, take a look at Mozilla’s Knowledge Base entry on the subject. In general, if you don’t know what something is going to do, leave it alone.

This option is probably best suited for those who want absolute control over their browser, so I would suggest casual users leave the options as they are.