Unable to install my own add-on file

Hi. I want to learn how to develope firefox add-ons. So, i wanted to inspect example add-ons. I downloaded example repos from https://github.com/mdn/webextensions-examples . I choosed “borderify” add-on. As you know, it has manifest.json and js file. I packaged it “zip” file with 7-zip. (used store comparassion method)and renamed it, zip to xpi.

After that, i can install it on about:debugging without problem. But when i try install from extension menu, “Install add-on from a file”, it warns me with this message: “this add-on could not be installed because it appears to be corrupt”

Where am i doing wrong?

Add-ons must be signed by addons.mozilla.org in order for the standard version of Firefox to install them, but that requirement doesn’t apply to the Developer edition. See this related thread for more info.