That is a O/S controlled feature that sets up the mailto option so your browser can see that. You will have to look at a Windows forum for this setup or look online on how to do mailto setup.

I am able to find the mailto setting easily. However, there are no clients listed to choose from, no gmail, yahoo, nothing. If I click "choose other" I'm expected to choose an application from my computer. I just want to be able to click a mailto link and have it go out through a new tab via my existing, free, yahoo mail account.

https://support.mozilla.org/en-US/kb/change-program-used-open-email-links

Hi James Craw, in recent updates to Firefox, usually Gmail is listed but Yahoo is not. If you don't find another solution and you trust me, you can download an older handlers.json file and replace yours with that one. This thread has the info:

https://support.mozilla.org/en-US/questions/1374655#answer-1498538