I got sick of Thunderbird opening up links in Mozilla instead of Firefox. A friend mentioned that the preference responsible for opening up links was 'network.protocol-handler.app.http'. I searched around in the Thunderbird pref files for what it was currently using. On my machine it was set to launch 'x-www-browser'. This is actually a symlink into Debian's alternatives directory, so a simple

update-alternatives --set x-www-browser /usr/bin/firefox

did the trick.