Same problem here.

Tried following https://support.mozilla.org/en-US/kb/change-program-used-open-email-links https://support.mozilla.org/en-US/questions/1256448

Have renamed handlers.json. No file found named "mimeTypes.rdf". Also set windows default app for email to Firefox.

Then clicked an email address and looked in settings The mailto option is back but no option to choose gmail.

In the current file handlers.json I see: "mailto":{"action":4,"ask":true}},

In an old file now named handlers.json-1.corrupt I can see the following: "mailto":{"stubEntry":true,"handlers":[null,{"name":"Yahoo! Mail","uriTemplate":"https://compose.mail.yahoo.com/?To=%s"},{"name":"Gmail","uriTemplate":"https://mail.google.com/mail/?extsrc=mailto\u0026url=%s"}]},

You can use the Profile Manager to create a new profile and copy handlers.json from the new profile to your current Firefox profile folder to see if that restores the choices in the mailto action in Applications. It is not the intention to keep using the new profile, so you can remove it and only use it to get a fresh handlers.json.

Thank you cor-el! That worked.

I created a new profile and named it "...for mailto..." so I could recognise it. I opened Firefox so that it populated the profile with the handlers.json file. Found the text needed: "mailto":{"stubEntry":true,"handlers":[null,{"name":"Gmail","uriTemplate":"https://mail.google.com/mail/?extsrc=mailto&url=%s"}]}

pasted it into the handlers.json of existing profile.

So far so good... However, lessons for those who try it:

Then I got a little confused as Firefox kept opening the non-default "...mailto..." profile. Tried renaming the folder of the "...mailto..." profile "temp ..." but then Firefox wouldn't open at all. Renamed it back. Used about:profiles to tell Firefox to set the correct profile as default (which it appeared to be in about:profiles but wasn't using when I opened Firefox normally).

Found the gmail option - success.

Thanks very much.