html page mail link to webmail program

[Ethan Furman]

Just in case that subject line is not perfectly clear:  ;)

My wife (using a Win7 machine) will be on a web page that has a link to mail somebody.  She clicks on it, and it opens 
the currently installed but unused Thunderbird.

Ideally, what would happen is a new window/tab would open to gmail with a new compose window with the email address in 
place and the cursor in the subject line.

Is this already done somewhere?  If not, any ideas on which libs/packages to use to make it happen?

--
~Ethan~