This was taken from: http://gnome-hacks.jodrell.net/hacks.html?id=11

1. Type about:config in the address box
2. Right-click and select New->String
3. Name "network.protocol-handler.app.mailto" without the quotes. Value =
"youremailclient" I used "evolution" (again, without quotes).
4. Right-click and select New->Boolean
5. Name "network.protocol-handler.external.mailto" without the quotes. Value =
"true" (again, without quotes).