So when you click a mailto link Firefox behaves as though you did nothing? First, could you check that Thunderbird is selected as the application to handle mailto links in Options. This article has the steps: [[Change the program used to open email links]]. If that is already right, it could be a problem with the file that stores those application preferences. 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. You will need to restore some settings such as your preferred PDF viewer. Any improvement with mailto links?