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'');

For additional information, please review the content on Mozilla's "Hyperlinks in messages do not work" webpage.