I recently upgraded from 12.04 to 13.10 and links in Thunderbird don't work, i.e. don't launch or open in Firefox. I have double checked that Firefox is still the default browser:
>> sudo update-alternatives --config x-www-browser
There are 2 choices for the alternative x-www-browser (providing /usr/bin/x-www-browser).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/bin/google-chrome-stable 200 auto mode
* 1 /usr/bin/firefox 40 manual mode
2 /usr/bin/google-chrome-stable 200 manual mode
I have checked it is not an add-on by running Thunderbird in safe-mode. In the config editor of Thunderbird I have checked that 'network.protocol-handler.external-default' is set to 'true'. I have also added 'network.protocol-handler.app.http' to the config (linking to /usr/bin/firefox) to no avail.