Well then tell Firefox that. You can configure what it's behaviour is in the preferences pane. The defaults are perfect, it's your preference that's different from the standard. Which is why they put it in Preferences.
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.
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!