As simple as adding a new setting in your about:config :-

1. Open Firefox, type "about:config" (no quotes) in the address window, and click enter
2. Right click on the window and choose 'New', then 'String' from the pop-up menu that appears
3. In the first pop-up box, enter: "network.protocol-handler.app.mailto" (no quotes, and it might just be easier to cut 'n paste this into the box)
4. In the next pop-up box enter the path to Thunderbird, or another email client (e.g. "/usr/bin/mozilla-thunderbird" - I entered "/usr/bin/claws-mail" without the quotes, of course)

[source]