1. Open the Thunderbird email program and wait until it is idle.
2. Click on Tools > Options to open the settings. If you only have a single Thunderbird button click on the button and then on Options there.
3. Switch to Advanced > General and click on Config Editor here.
4. Filter for the term network.protocol-handler.external-default by copying and pasting it into the search field at the top.
5. A value of false indicates that Thunderbird won’t open links in external clients.
6. Double-click the value to set it to true.
   
7. Keep the editor open for now.

The change takes effect immediately and you should be able to open links in the default browser from that moment in time on. Note that this will only fix the issue if the value of the parameter was set to false initially.

If that is not working, try the following option.

1. Go back to the editor
2. Filter for the term network.protocol-handler.warn-external.http and set it to true with a double-click (it is false by default)
3. This displays the “open with” dialogue when you click on a link for the first time which you can use to pick a program for the protocol. If you want to always open links in the application check the remember my choice for HTTP links at the bottom of that screen.
4. Repeat the procedure for network.protocol-handler.warn-external.https and network.protocol-handler.warn-external.ftp to display the prompt for HTTP and FTP links as well.

The parameters should be enough to resolve the links not opening issue in Thunderbird.