I installed acroread from the repositories, and it has decided it should be the default plugin in Firefox, such that when I click on a pdf, it opens in a Firefox window. Unfortunately, it does not display (at least when in dual head mode).

This should be simple, but I cannot disable the plugin. Under Firefox Edit -> Preferences-> Applications -> pdf is set to "always ask" so I don't know why it uses acroread in the first place. Under Tools -> Manage Content Plugins I see acroread is selected for pdf content. I have the option in the dropdown to search for a new plugin, but not to simply download the file.

Typing about:config into the Firefox toolbar gives me a list of configuration options, but does not mention plugin.disable or acroread anywhere.

How do I disable the plugin? I have Firefox 11.0 on Ubuntu 11.10