So after over a month of it not working and just checking not ten minutes ago, as soon as I posed a question here it started working again. I don't know if it is fixed, intermittent or what but I do know it is very frustrating. EDIT-Added info When I tested just after posting initially, Thunderbird was closed. After I posted this time I opened Thunderbird and again it wouldn't function. In Summary; If Thunderbird is open, mailto: links do not work. If Thunderbird is closed, mailto: links open a new write window.

As stated above, it is frustrating.

Do you have TB selected for mailto under W10 Settings, 'Choose default apps by protocol' and in Firefox Settings/General/Applications?

I probably should have mentioned that Thunderbird is set as default mail client, default for mailto: protocol, and all Thunderbird settings are set as required. To me that was obvious so I didn't think to mention them. I'm too close to the issue I guess. As stated above, it works if Thunderbird is closed, however if it is open and minimized it doesn't. That would indicate that all settings are correct or it would not work in any event. I can work around it, but I don't like unanswered questions. They make me fidgety.

It's not exactly the same problem, but MAPI issues can sometimes be fixed by running the installer for the same version and restarting the computer. Try that and see if it helps.

https://support.mozilla.org/en-US/questions/1323685

I had seen that but thought it was a different problem and passed on it. I just now figured nothing to lose so I downloaded a fresh copy of 78.12.0 and ran the installer and it did indeed fix the problem. Hopefully it is a permanent fix. Thanks for the assist.

AAAAnd when I checked again this AM it had reverted to the previous stat of NOT working. Period. Did I say this was frustrating? I once again re-installed and it started working again.

Do you have other browsers installed besides the default? You might be able to make the setting 'stick' by choosing another app in the W10 protocol and default settings, as well as the mailto setting in all browsers, restarting the computer, then switching all the settings back to TB.

I have tried other browsers, changing the mailto: protocol to Outlook, running in safe mode in all my browsers. The results are always identical to each other. It is currently functioning intermittently so I will wait for a new Thunderbird update and see if anything changes. Thanks

There will soon be a minor update in the 78 branch, but when the new version 91 is released (fall 2021), I recommend creating a new profile for that version. It's possible to run different versions of TB, each with its own profile.

It wouldn't surprise me if the mailto problem is caused by having Outlook on the same computer.

Outlook was not installed when the problem first manifested. Installed only for testing.