As of the recent update of Firefox from 38 to 45, the esteid-browser-plugin is not digitally signed .. can't be verified by Firefox 45.2.0.
Thus, it is disabled. Probably the developers of the plugin will soon update its signature.
If you still want to use it, you can disable the digital signatures (not recomended by Mozilla) of the extensions by going in about:config of Firefox
and changing the variable xpinstall.signatures.required to false.
Astor