Changing the web browser invoked by Thunderbird
From MozillaZine Knowledge Base
When you click on a web link in Thunderbird, it invokes the default web browser for the operating system. You could change the default browser for your operating system but that effects other applications. Usually the best solution is to specify what browser Thunderbird can use with the network.protocol-handler.app.http setting. While that setting is normally associated with Linux, it will work under Windows and OS X too.
You could edit the prefs.js file and add user_pref("network.protocol-handler.app.http", "/usr/bin/firefox"); (replacing /usr/bin/firefox with the appropriate path) or use the config editor to add that setting. Use Tools -> Options -> Advanced -> General -> Config editor to launch the config editor, right click on its listing, select New, select String, enter network.protocol-handler.app.http and then the complete path to whatever browser you want to use. If the setting already exists just double click on the setting to change the path.
Its also possible to add command line arguments such as specifying private browsing mode and open the web page in a new tab: "C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -private -new-tab
If the web link uses a secure connection (https) you also need to add/edit network.protocol-handler.app.https.