Have you tried it in Firefox? If yes see my other comment:
For Firefox try creating a new zip file after you unzipped the other and then add the content of the 'hive-keychain-master'-folder to that zip file (not the folder itself, only the content). Then try installing via 'Install Add-On from File'.
I can't get this to work in my Firefox but it works in the debugging modus. Maybe it works normally for you?
I'm using Brave not Firefox, so can't help there! I'll try it tomorrow.
Hi - with the cut and paste into a different file, it worked just fine!
Good!