XPI files are installable Firefox extension files that have everything you need to get an extension up and running in your browser. The average user isn’t likely to be bothered about these files but extension developers who are looking to turn their creations in to a simple XPI file or ones who would like to create one out of an existing extension and take it apart might be interested in Zippy Extension Packager. It’s an extension for Firefox that packages installed extensions in to an XPI file.
The extension adds a Create XPI option in the context menu when you right click an extension in the Add on Manager. Clicking it opens a new page with options to create a chrome JAR. If the extension was installed as an XPI file however, the extension will not create another one.
Check the respective boxes if you want to include amo.properties files and if you want to create chrome JAR files and click the Create XPI. The extension will list the contents of the file as well as it’s location and ID. The extension is useful mostly for developers but other users can use it to create a backup of all extensions installed in Firefox if ever they want to uninstall Firefox (browsing history, profile and all) but don’t want to go to the trouble of reinstalling each one by going online.