The Google toolbar already has a built in feature that lets you make Gmail as your default email client. But if you don’t have the google toolbar installed this is how you can do it in the latest version of Firefox.
associate “mailto†links in web pages with Gmail, so that when you click on e.g. info@seocompany.ca you’ll be instantly forwarded to create a new message in Google’s email client.
How to Activate Gmail in Firefox
Fire up Firefox, Open Tools -> Options -> Applications, and look for the entry titled “mailtoâ€. In the dropdown box, pick Gmail. Thats all!