An XPI file is a compressed installation archive used by various Mozilla applications, including Firefox, Thunderbird, and SeaMonkey. It contains software, such as a web browser plugin, extension, or theme. XPI files also include an install.js or install.rdf file, which stores the installation instructions.

Some versions of Firefox allow you to install XPI files may by simply dragging and dropping the file onto an open web browser window. The Mozilla software uses a component called XPInstall, (which includes Mozilla installer script) to install the XPI packages. If dragging and dropping the XPI file does not work, you can install the file in Firefox following the instructions below: In Windows, select the Menu (three horizontal line) icon in the upper-right corner and click Add-ons (select Tools → Add-ons in macOS). Select Extensions, Themes, or Plugins. Click the gear icon dropdown menu and select Install Add-on From File.... Navigate to and choose your XPI file and click Open. Here are instructions for installing an XPI file in Thunderbird: In Windows, select the Menu (three horizontal lines) icon, click Add-ons, then Add-ons again (select Tools → Add-ons in macOS). Select Extensions, Themes, or Plugins. Click the gear icon dropdown menu and select Install Add-on From File.... Navigate to and choose your XPI file and click Open. Since an XPI file is Zip-compressed, you can manually extract the contents in the file by renaming the .xpi extension to .zip then extracting the contents with a Zip compression utility, such as Corel WinZip, 7-Zip, or Apple Archive Utility. You most likely will find a .JAR file, which is typically used for containing the software to install, and an install.js or install.rdf file.