Hi,
Required add-on signing for extensions is new in Firefox version 43. Back in February 2015 it was announced that, for a safer add-on experience, all extensions will have to be verified and signed by Mozilla before they can be installed. More insight into that decision is contained in Mozilla's April 2015 blog post.
Extensions hosted on addons.mozilla.org are automatically signed. If an extension you wish to use is distributed by a third-party, it must be signed before you can install it. Unsigned extensions that you have already installed will be automatically disabled. Please ask the add-on developer or vendor if an updated and signed version of the add-on is available. You can also visit addons.mozilla.org to see if there's a signed version of the add-on you can install.
This signing requirement is mandatory in regular release and beta versions of Firefox. For testing purposes, it will still be possible to configure Firefox Developer Edition and Nightly versions to run unsigned extensions. For more information, please see Add-on signing in Firefox and https://wiki.mozilla.org/Add-ons/Extension_Signing.
You can visit the mozilla.addons.user-experience group to further discuss add-on signing.
