Firefox has a lot of settings, most of which are hidden from view as they can quite easily break the browser or make everything behave quite oddly. There's almost no way of stumbling across how to change these settings by accident which is likely why they are hidden away in this fashion. To access the hidden configuration options…
Firstly open up Firefox
and then type about:config into the address bar
and press Enter
You should now be looking at a rather strange warning message. Tick the box
then click the option given
You'll now find yourself looking at pages and pages of configuration options. Be sure you know what you are doing before changing any of them as they can break your browser (as advised by the warning messages you saw earlier!).