The great thing about Firefox is that it has a built-in option to set mailto links to open up in GMail. That means you don’t have to stick with Outlook or Thunderbird or any of the desktop programs. To change your mailto handler in Firefox, just go to Tools > Options > Applications tab, and then type “mailto” into the search box. Next, click on the drop-down menu and select Gmail. Now, any hyperlinked email address that you click in Firefox will be opened in Gmail automatically!
How To Open All Mailto Links With GMail Automatically in Firefox