Are you running the packaged Firefox for linux from Mozilla or another pre-packaged by a third party? To answer your question, there is a list of default mime types. In the profile however there is mimeTypes.rdf where a file is labeled. Deleting this file to delete a bad entry is how to reset it. However if this is happening by default, the extra quotes may need to be removed manually (with code.) However please do no hesitate to open a bugzilla bug as well at bugzilla.mozilla.org with an example. "Note: You may get problems if you download a file that is labeled with the wrong Internet media type. There is no way to identify or delete a bad entry in the list. If this happens you may need to look at the file mimeTypes.rdf in your Firefox profile folder. Deleting the mimeTypes.rdf will reset all download actions to the default settings."