Pop Up Saying I Have Viruses?

Alright, Well I Was on Mozilla Firefox and all of a sudden This Visa Security thing popped up saying I had like 30 viruses. I clicked allow Visa security on accident. I Went to sleep and the next morning it said "System Hijacked" On the corner of the page. Is someone hacking me? My dad is doing a scan right now to see whats going on. Please Help!