Nice easy tip this one.
Open Firefox, and open up GMail/Googlemail.
Paste this command into the address bar with the GMail/Googlemail page on view
and click on green arrow
Firefox will ask if you want to add the application, to which you say yes.
Now click on a mailto link, and Firefox will ask you which email client you wish to use. Select Googlemail and also tick the box to allow this to persist.