Links not opening in the browser or two windows opening on click

If links are not opening up in the browser, or if two tabs/windows are opening on each click, here are few things to try:

  1. Change your default browser to another browser and then change it back. In some scenarios after a browser update, the OS setting for the default browser fails to get updated.
  2. Use the Postbox Config Editor and make sure the hidden network.protocol-handler.external-default preference is set to true.
  3. Use the Postbox Config Editor and make sure the hidden network.protocol-handler.warn-external.http preference is set to false. Certain add ons (like ThunderBrowse) can leave these hidden prefs in the wrong state after uninstall.
If these steps don't help, it's recommended to reinstall ThunderBrowse, change a preference, and then uninstall it again. Here's how:
  1. Go to the ThunderBrowse website and download the latest version: 
  2. Open Postbox, and go to Tools > Add-ons, and click the action button at the top right corner to install it. 
  3. Once it's installed and you have rebooted Postbox, go back to Tools > Add-ons and click the Preferences button next to ThunderBrowse 
  4. In the resulting window, click the "Links" tab, and check the option to "Disable ThunderBrowse link loading" 
  5. Click a link and verify that it now loads in your default browser. 
  6. Disable/Uninstall ThunderBrowse, and restart Postbox to make sure links still open in your default browser