Disable add-on signing in Firefox Nightly and Developer streams. It’s a norm from Firefox 40 version that all add-ons you’re going to install in Firefox browser should be signed and verified by Mozilla, that means you can no longer install unsigned third-party add-ons in Nightly, Developer, Beta and Stable channels.
For more details, read: Mozilla introduces extension Signing for Firefox
When you try to install an unverified add-on, you’ll get this error or warning: ‘Nightly has prevented this site from installing an unverified add-on’.
Are you a developer or advanced user using Nightly or developer channel? Mozilla lets you disable signature checking for add-ons through a setting in about:config.
How to install unsigned third-party add-ons in Nightly or developer without signature checking
1. Visit about:config
2. Toggle xpinstall.signatures.required preference value to ‘false’.
After doing this, you can able to install the add-on after going through two warnings and that’s normal: first you need to ‘Allow’ it, next, read the caution prompt and click ‘install’ button.