Firefox 3.5: Set Gmail as the Default Email Client
Contributed by shamanstears 
July 8, 2009 
Tagged: Mozilla Firefox
If you use Gmail as your primary email client, Firefox 3.5 makes it easy to associate any mailto links that you might encounter to us Gmail as the default email client. While this has been possible with previous versions, you don’t need to jump through any hoops or use any add-ons with Firefox 3.5.
1. Open Firefox.
2. Go to the menu, click Tools and select Options.
3. Click the Applications tab.
4. Scroll down to the mailto Content type.
5. Select the associated Action dropdown and select Use Gmail.
6. Click the OK button.