you have to install an application that reads XPI files.
The XPI file type is primarily associated with 'Mozilla/Firefox Browser' by Mozilla.
This is basically a ZIP file that, when opened by the browser utility, installs a browser extension.
To install a local version of the file, you can drag and drop the file onto an open Firefox browser window or use the Firefox File | Open menu command. In most cases the file will be downloaded via Firefox after you've searched for new add-ons and will, at that time, automatically install itself and be available after a browser restart.
This extension applies to both Mozilla and Firefox browsers