When I click on a link in Thunderbird, I get the error: 'Error: The browser specified in Preferences ->Preferred Applications does not exist'

First, quit Thunderbird (or else any changes you make to the file below will be overwritten)
Add the following line to your prefs.js file in your .thunderbird directory:

user_pref(''network.protocol-handler.app.http'', ''/usr/bin/firefox'');

Click on the link below for more information:
http://www.mozilla.org/projects/thunderbird/linuxurls.html