How can I install a downloaded Firefox add-on?

I have downloaded the BetterPrivacy Firefox add-on to disk (file name "betterprivacy-1.29-fx.xpi", 24 KB) outside of Firefox (Windows).

How do I install this add-on into Firefox?

(I know that the downloading and installing can be done from within Firefox, but assume for now that this possibility does not exist.)

Just use the File → Open File... menu item to open the .xpi file and Firefox will install the add-on.

Just drag & drop the file into Firefox.

Or you can simply drag the .xpi file into the Add-ons window.

Or right click on the XPI file → Open with → Firefox.

There are plenty ways to skin that cat :)