You would need to reset firefox.

PLEASE SEE: https://support.mozilla.org/en-US/questions/965956 - I have ransomware on my Firefox operating on a Mac, how do I get it off

Readers of this thread may be interested in a related thread about a scam using FBI or Europol names and site addresses. It is in fact not a virus and very easy to deal with. There is nothing much to remove and it can be dealt with in seconds see

• WARNING FBI LOCKED BROWSER!!! /questions/981475#answer-516676