Official Gmail Blog: Tip: Set Gmail as your default email client in Firefox 3 - http://gmailblog.blogspot.com/2008...
Instructions didn't quite work, but I managed to find the bug at: http://digiwanderlust.blogspot.com/2008... - needed to set gecko.handlerService.allowRegisterFromDifferentHost to true in about:config - Arvind Sundararajan