You are probably already aware of how to use Settings. https://support.mozilla.org/en-US/kb/change-program-used-open-email-links

If you can't select it there, you will have to follow the procedure outlined in the answer below. https://support.mozilla.org/en-US/questions/1305068#answer-1351231

When did you set up the new desktop? Yahoo was in there a few months ago, so it would be a very recent change if it's missing.

Could you verify that the compose address still works:

https://compose.mail.yahoo.com/?To=example@example.com

Yes...the 'compose.mail.yahoo.com/?To=%s' goes directly to my yahoo compose email as designed. As far as the missing mailto apps, I might have deleted them like I did "Gmail" by accident. I also am 'missing' some application filetypes next to the mailto type. The desktop is a new gaming (first time for me) Alienware Aurora R14. I was trying to set it up by stripping a lot of Microsoft software such as OneDrive. It is for my wife and me in our "golden?" years...I'll look into terry21's suggestion...but not today...it's my young wife's 72nd birthday.

PS: I'm running Firefox 98.0.1 now and I started using Firefox at release 8 (before that it was Netscape).

If one of your other PCs is still running Firefox, perhaps you could transplant the handlers.json file from that Firefox's profile folder to your new one to carry over the old Applications list to get a head start.

General article on profile contents: Recover important data from an old profile

Ok that worked. I had tried to find where this information was stored, but nobody seemed to know except you. Thank you. I renamed 'handlers.json' on the desktop, put the copy from our travel laptop in the same location and then brought up Firefox. Presto! Voila! I now have yahoo mail choice back in my mailto list on this new desktop. Stiil don't know what happened...maybe these old fingers pulled a 'fat' finger... Thanks for coming to the rescue.