A suspicious popup came up on Firefox, but as soon as I clicked the red X to close it, another message popped up and asked if I was sure I wanted to continue closing the window. The options were "ok" to continue, or "cancel" to stop. I was in such a hurry I didn't see what the popup was for, but this was the first time I've been prompted to click another option to close the window, so I am suspicious of what it could possibly be. I clicked cancel and the window immediately closed. I have never had a questionable popup such as this in the two years I've had my Mac, so I was curious as to whether I should be concerned. I've restarted Firefox and deleted all browsing history/cookies. Is it possible this is malware or a virus?