1. Download the xorkeesign Thunderbird add-on to your PC

2. Open Thunderbird and click the "Settings" menu

3. Select the "Add-ons and Themes" menu which will open the "Add-ons Manager"

4. Click the "Settings" icon under the "Manage Your Extensions" section

5. Select the option "Install Add-on from File..."

6. Browse and select the .XPI file that was downloaded to your PC

7. Thunderbird will prompt to install the extension. Click "Add"

8. Your are done. Close the "Add-ons Manager" and continue using Thunderbird