If you do anything with a PDF file in the Open dialog, Firefox becomes unable to open locally saved PDFs using PDF Viewer (pdf.js). The Open dialog is often shown when the MIME type of the file is incorrect, but sometimes the Open dialog is shown even if the MIME type is correctly application/pdf (example: http://vexillium.org/dl.php?/Windows_Kernel-mode_GS_Cookies_subverted.pdf ). If the MIME type is incorrect, Firefox permanently becomes unable to open locally saved PDF files with PDF Viewer. A new content type entry is created in Options -> Applications, which cannot be deleted in the UI. The user has to use the Reset Firefox button to recover, unless he knows how to edit mimeTypes.rdf. If the MIME type is correct but the Open dialog is shown anyways (as with http://vexillium.org/dl.php?/Windows_Kernel-mode_GS_Cookies_subverted.pdf ), if you do anything with the file, the action for the file is automatically set to "Ask always", which makes Firefox unable to open any PDF file directly in PDF Viewer until the user changes the action back to "Preview in Firefox". This needs to be done every and every time when you open a PDF file which opens the Open dialog even though its MIME type is correct. These problems don't happen in the add-on version of PDF Viewer.

Expected results: Unless "Do this automatically for files like this from now on" is selected, no permanent changes to the content type handlers should be done. It's unlikely that an user wants to change the handler to "ask always" or something else just because some website forces the file to be saved locally.