How to Open the File Extension XPI
XPI files contain add-ons for Mozilla products, most commonly the Firefox Web browser. When you install browser add-ons directly from Mozilla's website, Firefox will add them automatically, rather than saving an XPI file. If you download an add-on elsewhere, however, you may need to open the file in Firefox manually to add the extension to the browser. You can also extract XPI files to view their contents without installing them.
Installing XPI Files
Double-clicking an XPI file will not automatically install it in your browser. To install an add-on, open a Firefox window and drag the XPI file from your computer into the window. Firefox will prompt you to confirm you trust the extension before enabling it. If you want to install an add-on for every Firefox profile simultaneously, place the XPI file in the Extensions folder within your Firefox installation directory.
Checking XPI File Contents
To check or modify the files contained within an XPI installer, open it as you would a ZIP file by using a program such as WinRAR or 7-Zip. If you change the file's extension from XPI to ZIP, you can even open it directly in Windows Explorer. You do not need to open an XPI file in this way to install it -- only if you're a developer looking to modify the add-on.
Information in this article applies to Windows 8 and Windows 7. It may vary slightly or significantly with other versions or products.