Setting @firefox@twitter.com + @gmail@twitter.com as your default mail program in @Windows@twitter.com required EDITING THE REGISTRY! The instructions at https://answers.microsoft.com/en-us/windows/forum/windows_10-other_settings/how-to-set-firefox-gmail-as-windows-default/296816f4-e2d7-48dd-ae71-84d18a02dc18 are almost correct, but of course everything has moved, so you'll dig for the analogous entry that makes it work for @googlechrome@twitter.com.
This was a challenge for me and I have a PhD in computer science. I don't know how normal people deal with this stuff. I guess they just pick one ecosystem and surrender.
@peterdrake I use Linux instead of Windows. That's how I deal. Of course I'm maybe not a normal person.
... and after all that, @firefox@twitter.com STILL isn't listed in the sharing menu from Screen Sketch on the @surface@twitter.com (although the default Windows mail program is).