It should let you install new, but Firefox won’t immediately re-verify the existing disabled extensions. It would be approximately 24 hours after the time stamp in this preference in about:config
To trigger re-verification sooner, you can right-click > Reset that preference, then do a normal Exit/Quit and restart of Firefox. Within about 60 seconds that should run and validly signed extensions should be re-enabled.
Also, I don’t know what a .prem file is, so you might get the certificate from a Mozilla site I’ve linked over here: https://www.jeffersonscher.com/sumo/intermediate-cert.html