Hi djmb, See if something in this article will help - Change the program used to open email links

If not, you may have to rebuild your Applications Handler - Type (or paste) about:support in the address bar and press Enter under Application Basics next to Profile Folder click the Open Folder button Close Firefox.

In your Profile Folder rename the file handlers.json to handlers.json.bak

Close the folder window and launch Firefox. Your Applications handler will be reset to it's defaults.

Many thanks for your reply.

Eventually found my handlers.json files, all five of them are corrupt. Renamed the first one handlers.json.bak to see if it made any difference and it became an unreadable Adobe Acrobat file. Feeling rather lost now.

Later. I've refreshed Firefox which seems to have solved the problem, Gmail is back as a Mailto option and I just have one Handler File with the corrupt ones apparently deleted.

Thanks for your advice, I learn something new every day.

Many thanks for your reply. I've refreshed Firefox which seems to have solved the problem, Gmail is back as a Mailto option and I just have one Handler File with the corrupt ones apparently deleted.

Thanks for letting us know that a Firefox Refresh resolved the problem. I'll mark it as the solution.

To RobertJ: Deleting _{or renaming} handlers.json is no longer a solution since it removes the mailto Content Type since at least Firefox version 98. See https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file/discuss/10697 for details.