This article explains what XPI files are, what apps use them, and how to open them in any program.
How do I install Firebug and Firepath? Including .xpi files!
An abbreviation for Cross-Platform Install (or XPInstall). A file with the file extension XPI (pronounced "zippy") is a compressed installation archive used to extend the functionality of Mozilla products such as Firefox, SeaMonkey, and Thunderbird.
An XPI file is basically just a renamed ZIP file that the program can use to install the add-on, be it a theme, plugin, etc. They can contain images and JS, MANIFEST, RDF, and CSS files, as well as multiple folders full of other data.
Firefox uses XPI files to provide extensibility within the browser. Drag the file into an open Firefox window to install it (select Add when prompted).
