How do I set firefox non-gmail webmail as the windows 10 system MAPI client

I would like to set the Windows 10 MAPI client to firefox, such that it opens my preferred webmail to send a new message. This is not technically difficult, however…

This is surprisingly hard to search for on the internet. No, dear hyper-optimized search results for the common case, I do NOT want

  • to set the mail client in the browser for mailto: links on web pages (I already set that).
  • to set the outgoing mail client in firefox (ironic)
  • to set the MAPI client to an installed program other than firefox, eg a mail client; I want to set it to firefox.
  • to set Thunderbird as the MAPI client; yes you are oh so clever to understand that firefox and thunderbird are both made by mozilla; but I typed exactly what I want: firefox, not thunderbird.
  • to set the default Windows Mail client as my MAPI client (I typed firefox)
  • to set Outlook as the MAPI client (I typed firefox)
  • to set the default mail program in Windows 10 settings (altho, if that worked, I would be happy)
  • to set the MAPI client to Chrome (I typed firefox)
  • to set the MAPI client to gmail (I said “webmail”, nothing about gmail)
  • to set the Windows ‘Sent To’ link in Windows shell

(The era of being able to search for specific things appears to be over. The search engines seem to rank popular answers higher than matches to your specific search terms. I assume that so many people provide sloppy search terms, perhaps because they don’t know exactly what they’re searching for, that the search engines assume that you don’t know what you’re searching for. Admittedly, that is me often. However, if you do know exactly what you’re searching for, God help you. Get a time machine and back to 1990, or get $1 billion and make your own google.)

The way I imagine this MAPI-firefox working is, I am in some program (Windows file explorer, Windows photo viewer, Microsoft Word, LibreOffice Writer, or many others) and I click that program’s “send to email recipient” or similar.

That program calls out to Windows 10 to invoke whatever MAPI client is set and requests a new message.

That MAPI client is firefox.

Firefox receives the request for a new message, and consults its mailto: link settings and picks the default to invoke that webmail link (with its parameters) to start a new webmail message.

If firefox can’t do that (but why not? doesn’t seem hard) then a 3rd party program that receives the MAPI request and opens the firefox mail:to link seems easy to write.

If it doesn’t exist, maybe I ought to write one.

But I can’t be the first person who wants this. It would have to already exist.