In today's earlier list of five extensions you won't need in Firefox 3, we said you won't need any special toolbars, third-party apps, Greasemonkey scripts, or extensions to get Firefox to use webapps to open certain types of links. This means that when you click on an email address that uses the standard mailto: email protocol, Firefox 3 itself can launch Gmail instead of a desktop app. By default, the Firefox RC 1 only comes with Yahoo! Mail as a possible mailto: link handler, which leaves Gmail users out in the cold—unless you know how to set it up by hand. Here's how to configure Firefox 3 to use Gmail as your default mailto: application handler.

Ready to set up Gmail? Roll up those sleeves.

1. Type about:config into Firefox 3's address bar. Firefox 3 will issue a warning about voiding your non-existent warranty. (Cute!) Click the "I'll be careful, I promise!" button to proceed.
2. Type gecko.handlerService into the Filter field until you locate the gecko.handlerService.allowRegisterFromDifferentHost key/value pair. Change it from false (its default setting) to true, as shown:
   
   Warning: This configuration change disables a security feature that ships with Firefox 3, which prevents your browser from registering link protocol handlers from a domain other than the one it's registering. Read more about what gecko.handlerService.allowRegisterFromDifferent Host does, and only proceed if you understand the risk. Don't worry, you can set the value right back to false when we're done.
3. Copy and paste the following snippet of Javascript into your address bar:
   javascript:window.navigator.registerProtocolHandler("mailto","https://mail.google.com/mail/?extsrc=mailto&url=%s","GMail")
   
   If you are a Google Apps user, use this code instead, but replace example.com with your domain name:
   javascript:window.navigator.registerProtocolHandler("mailto","https://mail.google.com/a/example.com/mail/?extsrc=mailto&url=%s","GMail")
4. Firefox 3 will ask permission to add Gmail as default mailto handler. Click the Add Application button, as shown.
   

Now, if you click a mailto: link—try the tips link on Lifehacker's sidebar—Firefox 3 will ask which application you want to use. Choose Gmail, and select "Remember my choice for mailto links" to set the preference permanently.

To be on the safe side, head back into about:config and set gecko.handlerService.allowRegisterFromDifferentHost back to false.

Firefox 3's new ability to register webapps as link handlers has lots of potential usage, for everything from fax and telephone numbers to IM buddy name links. Setting up your mailto handler is just the beginning. Read more about Firefox 3 web protocol handler possibilities. Thanks, readme!