Your browser/computer is infected with malware from another site. But, it's waiting until you are doing something else to pop-up the window. They don't want you to know where you got it.
Every FReeper should have 4 (free for home use) utilities
1) Avast Anti-virus
2)MalwareBytes
3)CCleaner
4)AdBlocker Plus
GET THEM NOW and scan/cleanup your system!
I had that happen one time...cleared out history cache and everything then rebooted...never got it again...
I did here these tend to hang out in history....