About a year ago I posted how to make gMail your default email client using Affixa. While that still remains a great way to do it system-wide, if you only want gMail to come up when you click a mailto: link in Firefox, I got this tip from the MaximumPC Feb 2010 issue. In Firefox, go to Tools > Options, and then select the Applications tab. Type “mailto” in top input field. From the drop down select gMail.