Sometimes Firefox will send an incorrect content type with a PDF, such as "application/download" instead of "application/pdf". You can rename or delete the settings file that stores these associations and have Firefox regenerate it. 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 Restart Firefox and content association should return to defaults. Note: Your PDF preference likely will be reset to the internal PDF viewer, but you can switch it to the Adobe plugin as described in this article: [[How to disable the built-in PDF viewer and use another viewer]]. Any improvement?