We have a signed add-on the will not load via deployment. We get a message "That the plugin could not be verified." We install the add-on in a corporate environment by registry keys. We followed the steps in this url to deploy to about 1700 endpoints. https://developer.mozilla.org/en-US/docs/Adding_Extensions_using_the_Windows_Registry The add-on will install manually by selecting the option "install add-on from a file".