XPI files are used and created by a variety of Mozilla applications, including the Thunderbird email application and the Firefox Web browser. The files that contain the .xpi extension are very similar to zip files. When they are opened the files "decompress" and install a browser extension into the Mozilla browser or Thunderbird application.
To install an XPI file a user can simply drag and drop the file into the interface of the application the file is associated with. Common add-ons that use the .xpi extension include the Alexa and PR add-on indicator and other plug-ins that are essential to the operation of the Mozilla software applications.
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File.org aims to be the go-to resource for file type- and related software information. We spend countless hours researching various file formats and software that can open, convert, create or otherwise work with those files.
If you have additional information about the XPI file format or software that uses files with the XPI suffix, please do get in touch - we would love hearing from you.