Last year, Firefox 3 added the option to associate protocols with web applications . A common way to link to mail addresses is using the mailto: protocol, so you'll often find links like security@google.com .
When Firefox 3 was released, Yahoo Mail was the only webmail service that could be selected directly and the instructions for adding Gmail were complicated . Gmail has been added to the list of mail applications and it's easy to set it as the default email client in Firefox and open all the mailto: links in Gmail, without installing extensions: