I recently noticed some odd error message, and found an open bug report in Bugzilla. The file in question is an XPI shipped with the Mozilla Firefox installer.
https://bugzilla.mozilla.org/show_bug.cgi?id=1315213
The file in question is in /browser/features/aushelper@mozilla.org.xpi (bootstrap.js, which originates in mozilla-central/browser/extensions/aushelper/bootstrap.js), which I simply wanted to try to edit to see if I could scratch an itch, without having to download and learn to set up a fully blown VS2015 build environment from scratch, and all dependencies, and compile from source, just to edit 2-3 lines in a plain text JS file.
Although I have had a little experience with add-on development, and am familiar with using an unzip program to extract the archive, I was unable to simply unzip this XPI, as it was unrecognized by 7-Zip.
What’s the deal with these XPIs? Can’t seem to read any of them (in browser/features) as archives.