My firefox screen is frozen by a message from "systemsecurity-pro" that my iphone has been hacked. I think this is a scam and don't want to tap the "OK" button.

Generally nothing can really happen if you click OK — please just avoid confirming any calls being triggered to phone numbers and similar. This dialog can only come from the site that's open in the tab underneath, and yes, it's completely bogus and most likely a part of a scam of sorts. If the dialog keeps repeating indefinitely and you can't close the actual tab triggering it, so it appears even after restart, there are some steps described in support.mozilla.org/questions/1594665 that you can try to get rid of the tab before it can load.