The procedure for firefox 3 worked for me with firefox 11.0. Here are the instructions I followed:
http://fastmail.wikia.com/wiki/Firefox_3_default_mailto
You may also want to check out the mailto_url page for the gory details:
http://fastmail.wikia.com/wiki/Mailto_url
Or if you need even more info here is the fm wiki search I used:
http://fastmail.wikia.com/wiki/index...ulltext=Search