You have downloaded a Thunderbird add-on on your PC and want to install it? Here's how to proceed:
- Open Thunderbird.
- Click on Tools > Add-ons
- The Add-ons Manager shall open.
- Click on the "Tools for add-ons" button (gear icon) > Install Add-on From File.
- Browse for your downloaded add-on (.xpi) and click on Open.
- Follow the on-screen instructions.