Yeah, as others have said this is just a common scare tactic used by malicious code on websites. It really is just a warning message popup. They hope that you'll click whatever link they put and then download whatever spyware/ransomware they are trying to push. Sometimes they just want to con you into paying a fee for a fake "virus clean."
Also, as others have said use Safari and a "content blocker" app. On iOS, there really is little benefit (other than the look of the browser) to using Firefox. All iOS browsers use the safari engine in the background to render pages, but lose many of the protections and integrations native Safari has.
On Android, I'd say absolutely Firefox is one of the best.
