For the fans of GMail here’s another tip.If you want to set GMail as default client in your favorite Firefox3 browser i.e., if you want your  Firefox to launch Gmail when you click on email address links then follow these tips by the  Lifehacker people:
1) First go to Gmail and sign in.
2) While in Gmail, copy and paste the following into your browser’s address bar and hit enter.
javascript:window.navigator.registerProtocolHandler(”mailto”,”https://mail.google.com/mail/
?extsrc=mailto&url=%s”,”Gmail”)

3)Then you will see the following dialog

4)Press Ok and Presto! you are done .