When I create a new blank Firefox profile, mailto is there by default, so perhaps at some point it got deleted from your download handler/application preferences file. That file can in theory be edited, but since the format is not well documented, it might be simplest to rename that one and have Firefox generate a new one. Here's how: Open your current Firefox settings (AKA Firefox profile) folder using Help > Troubleshooting Information > "Show Folder" button Switch back to Firefox and Exit Pause while Firefox finishes its cleanup, then rename '''mimeTypes.rdf''' to something like mimeTypes.old. Restart Firefox and check Options > Applications again. Any luck? ---- Also, your Firefox reported itself to the forum as version 17, which is no longer secure (Mozilla discloses [https://www.mozilla.org/security/known-vulnerabilities/firefox.html security flaws] after each new release). Is something holding you back from upgrading to Firefox 20? Please let us know so we can suggest solutions or workarounds. If Help > About Firefox shows Firefox 20.0.1, you may need to clear the preference that is misreporting your version number. See: [[Websites say that Firefox is outdated or incompatible even though it's the latest version]].

Worked, thank you for making my day better!