Since installing LMDE 201204 my Thunderbird does not open links in a browser unless the browser is already open.
I have tried various options:-
Changing the default browser using
sudo update-alternatives --config gnome-www-browser
Adding strings like network.protocol-handler.app.http = x-www-browser or /opt/firefox/firefox or /usr/bin/chromium
-at least these will open the links if the browser is already open (but the wife likes it the way Linuxmint 12 worked)