The preference pane does not let you create new lines -- the only way to add rules as a user is to encounter the Content-Type in the wild.
https://addons.mozilla.org/en-US/firefox/addon/8207
Awesome little gem: I went to Tools > Add-ons > Extensions > Find updates -- and it found and downloaded the new AMO version of my hacked local copy!
The preference pane does not let you create new lines -- the only way to add rules as a user is to encounter the Content-Type in the wild.