Chris,
A fuller description of how to edit config can be found here:
http://kb.mozillazine.org/Editing_configuration
Note, it describes "context click", I have no idea how to do this on a Mac, it may make sense to you.
Alternatively, you could just use this add-on
https://addons.mozilla.org/en-US/firefox/addon/6891
That way, there's no messing around changing this and that......
