Type about:config in the browser's address bar and hit enter. This loads Firefox's main configuration page.
Confirm the warning prompt if it appears.
Search for the preference xpinstall.signatures.required.
Double-click on it to toggle its value.
When you set it to false, you disable the add-on signing requirement.
Works for me. Google is your friend :)
See how we go in the future thanks for the prompt response and the info