Installing Add ons

After downloading a file (gcontactsync-2.0.12-sm+tb.xpi), W10 asks me what program do I want to use to open the file.

If you’re trying to install the add-on in Thunderbird, you can try looking for it in the Add-ons Manager in Thunderbird and installing it from there. Otherwise you can drag the downloaded file into the Thunderbird Add-ons Manager and that should begin the install process.