Download the free Google Talk chat application and open the options dialog. You'll see a general setting that says "Open GMail When I click on Email Links" - select that and exit.
This will set Gmail as your default mailto: handler. This means that when you click on any hyperlinked email address, a Gmail compose window will open in your default web browser. The To: address field will be prepopulated with email address.
This trick works if you are in IE - the GMail compose window would open inside Firefox.
To make this work inside Firfox, implement this small change:
1. Type about:config in the address bar.
2. Type mailto: and hit enter.
3. You'll see a setting called network.protocol-handler.external.mailto that is set to false. Double click it to toggle the value to true.
Your system will work just the way you want.