I think this problem is caused by an erroneous entry in one of Firefox's settings files. Since the file is difficult to edit, we generally suggest renaming it and letting Firefox start a new one. Here's how: Open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > "?" button > Troubleshooting Information * (menu bar) Help > Troubleshooting Information * type or paste about:support in the address bar and press Enter In the first table on the page, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer. Leaving that window open, switch back to Firefox and Exit, either: * "3-bar" menu button > "power" button * (menu bar) File > Exit Pause while Firefox finishes its cleanup, then rename '''mimeTypes.rdf''' to something like mimeTypes.old (in case you need to recover it later). Start Firefox back up again, and try the upload. You may also need to return to the Application options to select your preferred viewer for PDFs and your preferred handler for email links, if relevant: * [[How to disable the built-in PDF viewer and use another viewer]] * [[Change the program used to open email links]] Success? ''(Old thread on this: [https://support.mozilla.org/questions/1002615])''