. Whenever you get a message / popup like that;

DO NOT USE ANY OF THE PROVIDED LINKS OR ANY PHONE NUMBERS ! !

Such messages are only Spam that could lead you to Fake Support. Not only can they damage or hack your system, they charge you to do so.

You can report such a site at; Google Report Phishing Page {web link} which is the same when done while on site by going to Help > Report Web Forgery

Help us safeguard Mozilla’s trademarks by reporting misuse {web link}

I understand not to respond to scam calls or notices. What I want to know is: how can you tell if scammer has already compromised your browser or PC by locating the offending insertions? Can they get inside your browser or PC when you don't allow them access? Is the phone call claiming security problems just a random chance to get someone to bite?

Thousands of those type mailings get sent out all the time with the sender hoping someone is dumb enough to answer it.

Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.