The Firefox default way of handling mailto links for Yahoo email works okay if the mailto link only contains a "to=" key:value pair. More than that, and it totally fails to fill in any fields on the Yahoo compose mail page. I'm not sure who is at fault here, it appears to be Firefox, because I can manipulate the value being sent to the Yahoo compose mail URL, and it works. In fact, I wrote a small bash script to do just that.

The problem with more than one key:value pair is that Firefox fails to replace the ? with & after the email address in the mailto link, so after the argument is appended to the URL (second line of script, the default in Firefox for Yahoo) it winds up with 2 ? characters, which I'm guessing causes Yahoo to reject all the URL parameters. So my script essentially does what Firefox should do, by replacing that second ? in the URL with &. It doesn't appear to handle all characters properly, like apostrophes, but I didn't want to get that much into the weeds.

#!/bin/bash
arg="https://compose.mail.yahoo.com/?to="
arg+=$(/bin/echo $1 | /bin/sed s/?/'\&'/)
#echo $arg
firefox --new-tab "$arg"

You'll have to save this in a file and make it executable. Then in Preferences > General : Applications, modify the mailto action by choosing "Use other..." and browsing the bash script file. This will add the script to the list of email entries. I typically have this set to "Always Ask" for various email services.