XPI (pronounced as “zippy”) stands for XP Install or Cross-Platform Install. This is a zip file containing an install script and numerous data files. Files associated with this extension are basically used to give Mozilla Firefox browser updates. Thus, this file extension is but a ZIP file and when opened by Mozilla Firefox, it installs or updates a browser extension.
But XPI files can also be opened using any ZIP archive program by renaming the extension from XPI to ZIP first. Depending on how the ZIP archive program works, the user can then access the contents of the XPI file by unzipping the renamed file extension.
The most convenient way however to open a file with an XPI file extension is to double-click it and allow the computer system to decide which program or application can be used to open it. If nothing comes up, then the user can just drag the file to an open Firefox browser window and it will automatically install its content to it and the add-on or update will be available once the browser restarts.
Other Mozilla applications may also use this compressed installation archive and so it is not limited to Firefox alone. Thus, Mozilla Thunderbird and Mozilla SeaMonkey may also have primary components that can open this file extension.
Mac OS, Windows and Linux systems that are installed with Mozilla Firefox, Mozilla Thunderbird and Mozilla SeaMonkey are all capable of opening and installing a file with an XPI file extension.
