Hi Matt. First you need to find your profile. Go to about:support (type it in the address bar).
Close to the top is a blue column, Application Basics. About 6 lines down there is Profile Directory, besides this, in the grey area, is an Open Directory button. Click this and your profile folder will open. You need to create a file in this folder called user.js and paste the user_pref line referenced a couple of messages up into user.js, save it and restart FF and test.
If you prefer the cmd line, your profile is likely under X:\home, look in your config.sys for the SET MOZILLA_HOME line which should point at the root of your profile directory with your profile under Mozilla\Firefox