Thank you for letting me know. The reason the add-on uses the right-click method in the first place was that the built-in Yahoo! mailto was broken (and I didn't have a way to replace it).

Then the built-in mailto was working for a while, and then a few versions ago, Mozilla stopped including the compose URL for Yahoo! automatically in new installs, which would be relevant to you if you created a new profile or needed to delete your old handlers.json file to resolve a download action problem.

As far as I know, add-ons can't do anything about this. If I think of something, I'll do an update.

If you used that mailto frequently, you could consider downloading an older handlers.json file I shared on my website. Either you can use it as a reference to edit your own handlers.json file, or replace it. See: https://support.mozilla.org/en-US/questions/1374655#answer-1498538