iMac login displayed repeating Apple Security video with locked keyboard & mouse

It's a scam. Download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. 

Also download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

If you get a message that your system won't let Etrecheck run look for the link to older Etrecheck apps that will run.

I don't know about Firefox but in Safari to get rid of website popups do the following:

and uncheck the highlighted checkbox. Firefox may have something similar.