1. What is XPI file?
A file with the XPI file extension is a Mozilla/Firefox Browser Extension Archive file.
2. How to install XPI file?
2.1 Launch Mozilla Firefox application
2.2 Select "File > Open" from Firefox's menu bar. Browse for the XPI file on your hard drive. Click "Open" after selecting the file.
Or
2.2 Drag the XPI file you have downloaded on your computer to your browser window by holding down your left mouse button over the XPI file and using your mouse to drag the file directly into the middle of your browser screen and then letting go of your left mouse button.
2.3 Click "Install Now" after Firefox processes the file and recognizes it as an extension. It will take a second for Firefox to process the file, during which the "Install Now" button will be deactivated, displaying a countdown from three.
2.4 Quit Firefox and and reopen it to complete the installation of the extension.
2.5 Verify that the file is installed by clicking on "Tools" via the top of your Firefox broswer. You should then see the application on the list.
(If you cannot see Tools, please click Options, and make menu bar checked)