webmail.earthlink.net/newmessage?to=friend@example.com

I depends on how you interact with your mail. * If you are accessing Earthlink using an external program (such as Windows Live Mail, Thunderbird, or Outlook), then you would choose one of those from the drop-down. * If you are accessing Earthlink through their website, the way you add a new mailto destination is a bit involved. The first step is to figure out the compose link used by the webmail site. If you have any emails in your inbox with email links in them, you could see whether they have a standard URL format and test it with a different address. Let's say hypthetically, it's webmail.earthlink.net/newmessage?to=friend@example.com If you can create a new message simply by changing the email address, then there's a pretty good chance you can set up Firefox to send email addresses in other pages to that URL.