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?
Unsigned extensions are only supported on developer, nightly and ESR builds of Firefox https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox
Thanks for the info!
Unfortunately that method always says the file is corrupted, the same like before. Debugger is a little bit advanced for me, I think I'll just wait until dust settles, or maybe switch to Brave or Chrome.