I have successfully configured Firefox (43.0.4) to open mailto links in Gmail using Mozilla's instructions.
mailto now triggers a new Gmail Compose however this uses the current tab and navigates away from the current page.
This is particularly frustrating in Feedly when I want to send an article and then continue reading the next in my feed - I now have to navigate Back which reloads Feeldy, loosing the thread of where I was in my article list.
ctrl-clicking a mailto link does not work.
Is there any way to force Firefox to open mailto in a new tab or new window?