Not everyone running linux uses a desktop environment or window manager that supports a global “default email application” setting which Firefox and various other browsers are programmed to automatically detect. Sadly there doesn’t seem to be a preferences setting for this and it can get to be a real pain in the ass. For pre-1.5 versions of Firefox there was a patch against defaults/pref/firefox.js that set Thunderbird as the default mail client. I’m fairly certain with minor tweaking the patch could still work, but what if you don’t want to use Thunderbird?

To pick your own email client you need to tweak the internal Firefox config settings. First open a browser window and in the address bar type about:config. This will bring up the entire list of internal settings, but we only need the ones related to the mail application. In the filter bar type mail and press enter to filter out everything but mail related settings. First off we need to ensure that Firefox is exposing mailto links to external applications by checking these settings:

network.protocol-handler.expose.mailto default boolean true
network.protocol-handler.external.mailto default boolean true

If either of these settings don’t match the above on your system simply double-click it to toggle it between true and false. Once these two are set we need to add the handler for the default mail app. Right click anywhere in the window body, select new -> string. A window will pop up asking for the preference name. Enter network.protocol-handler.app.mailto for the name and /usr/bin/thunderbird for the value.

The value of this string is all the will determine the mail app that Firefox will use. It can be anything from Thunderbird to Kmail, even mutt if you’re like me.