Hello, there isn't an interactive way to edit the Applications (download handler) list. mailto really should be there by default. If no one has a better suggestion, you could rename the relevant settings file and have Firefox rebuild it. Here's how: Open your current Firefox settings (AKA Firefox profile) folder using either "3-bar" menu button > "?" Help > 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, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in File Explorer. Leaving that window open, switch back to Firefox and Exit, either: "3-bar" menu button > Exit/Quit (menu bar) File > Exit (or Firefox > Quit) Pause while Firefox finishes its cleanup, then rename handlers.json to something like handlersOLD.json. If you see a file named mimeTypes.rdf, rename that one to mimeTypesOLD.rdf. Start Firefox back up again. Is mailto back? In the following thread you will find more possible solutions for your problem: https://support.mozilla.org/en-US/questions/1256448