Sorry. It's in "Advanced Preferences". This will get you there (I just did it in my Firefox to make sure):
Type "about:config" in the browser's address bar. Hit "enter"
Click "Accept the Risk and Continue"
Check see if the value is "true". If the value is "false", click the button on the far right to toggle/reset it to true.
(of course, if it's already "true", do nothing, the problem lies elsewhere).