It should be one of the default options. As it is not there you can try resetting the download actions. You can do this by renaming the file that contains details of download actions, this will reset the list back to the default settings. # Open your profile folder, for details of how to access it see the [[profiles]] article # Close Firefox # In the profile folder rename mimeTypes.rdf to mimeTypes.rdf.old #Re-start Firefox and mailto should now be listed