Now you can use gmail as your default E-mail client for any kind of e-mail writing in Firefox by following two simple steps
Go to Tools => Options => Application Tab
and see for content type “mailto”.
Change this to Gmail, and now whenever you will click for a mailto protocol it will automatically open Gmail.
Try it over here by sending me feedback at firstname.lastname@example.org
Wishing you all my Best Wishes for Diwali