This one was driving me nuts and I finally found a solution. For several weeks, Thunderbird has not been opening links I click on them. I found a solution here: http://ubuntuforums.org/showthread.php?t=502651
- Open Thunderbird version 18.104.22.168
- Go to the Edit menu
- Click on Preferences
- Click the Advanced tab
- Click the Config Editor button.
- An about:config window will open up.
- In the Filter box, enter network.protocol-handler.app.http
- If the editor is unable to locate it, create a new string and input network.protocol-handler.app.http.
- Enter firefox for the string value.
- Close out and restart Thunderbird.
Hyperlinks should now be available in your email!