What content-type is being sent with the unviewable files/pages?''*'' Some content types such as text/html and text/plain are handled internally, and I'm not aware of any built-in way to modify that. There is a relatively old extension which you can use to create rules to override/replace the content-type sent by the server. I haven't tried it myself: https://addons.mozilla.org/firefox/addon/force-content-type/ ''* Edit: One method to determine this is to right-click the page after it loads and click View Page Info. The "Type" should be displayed on the first panel.''