Extension: xpi
What is it? Mozilla Installer Package
Where’s it from? Xpi files are what you download to install a Firefox add-on (a. k. a., “extension”). They are compressed archive files designed to work specifically with Firefox on all its operating system platforms. Updates to Firefox are also delivered via xpi files.
How do I use it? You can launch the installation of an xpi file just by dragging and dropping it into a Firefox browser window, or double-clicking on it.
Can I safely delete or move it? Yes
Can it be dangerous? Yes. Use antivirus software
–David Hakala