The screenshots below are taken from Mac OS X, but the process is the same on Windows.
Click into the Address Bar
The more advanced settings for Firefox aren't accessible from the standard "Settings" controls. You need to go to a special address to be able to change them. To do this; you'll need to type in a special url and then load it.
Start by clicking into the Address Bar at the top of your Firefox window.
The Address Bar is at the top of the Firefox window and usually shows the URL of the website you're looking at.
Visit the Advanced Configuration url
In the address bar, type in about:config and press enter. You do not need to put in http:// or www or anything like that; simply put about:config and press the Enter key on your keyboard.
Accept the warning about changing advanced settings
Because about:config will give you lots of advanced options which can potentially mess up Firefox, you will be warned about not changing anything you're not sure about. Click the "I'll be careful, I promise" button.
This warning will appear each time you visit about:config unless you untick the "Warn me when I attempt to access these preferences" checkbox. We recommend keeping this box ticked to remind you each time!
Can now search for advanced settings to change
Firefox has hundreds of advanced options which can be changed. To make it easy to find the ones that you are looking for, Firefox will give you a search feature.
There is a search box at the top of the page which you can use to search. Alternatively, you can click Show All to see a huge listing of all the options.