Is it popping up in front of what you're doing when you're working on the computer, or is this playing in the background? A website in an inactive (background) tab could be reloading itself at certain intervals... Does this occur even if Firefox is completely closed?

Well, some programs have viruses that have random ad popups. I say, look through your programs, and if there isn't one that you remember downloading, please uninstall it. If this doesn't work, I say, download Adblock Plus.

Next time, please include the kinds of advertisements.

Possible Mal-Ware Scan For Macs {web link}