title: Installation Error: "This add-on cannot be installed because it appears to be corrupted"
body: <p>I am currently trying to learn how to develop Firefox add-ons, but for my misfortune, I faced a problem before I even start doing so. I downloaded the demo add-on provided by the Firefox developer team which is called "Borderify" and can be seen here: <a href="https://github.com/mdn/webextensions-examples/tree/master/borderify" rel="nofollow noreferrer">GitHub</a>. After that, I opened WinRar, and converted the folder to a .XPI file. However, when I try to install it manually on my Firefox browser I got this message "This add-on cannot be installed because it appears to be corrupted". I will be grateful if someone points what I've done wrong.</p>