I've had this query for a while.
Your post made me finally search out how.
You need to set your browser as your default mail app.
Curiously to do this you go through Mac Mail. If you use a regular mail app like Thunderbird it can do it for you. It would be handy if FF could do this directly but I couldn't see how.
For Firefox which is what I use I booted Mac Mail and when it had finally imported junk from years ago - I gave up using it ages ago - I went to preferences -> General -> Default email reader.
This in the pull down choose Select ... and hunt down the app of your choice.
Anyhow, there may be other ways but that got it going for me.
Er... and Joel beat me to it.
Edited by Daniel Semler, 04 April 2014 - 11:53 PM.