Often I would suspect malware, but that is less likely on a Mac.

We do not know what you have installed on the Mac or even what Firefox extensions you have.Possibly some software bundled with a game related software will turn out to be the culprit.

Paste the information from the troubleshooting page into your next reply please. Maybe that will give some clues. ( If you run out of space edit out the print related information. )

• Navigate to about:support
• see Use the Troubleshooting Information page to help fix Firefox issues
• also see
  Troubleshoot Firefox issues caused by malware
  ( Unlike the Windows version, this does not offer advice and tips on scanning for malware)

When do you see these problems ? Is it only when certain websites sites have been opened.

stop pop-ups https://www.youtube.com/ watch?v= OkZmeigovCY

HARISH KUMAWAT, that was just a old video from July 2009 that slowly tells you on how to go to Tools->Options->Content (menus for Windows users and not Mac OSX) and making sure Block Pop-up Windows was checked which it is by default.

This article is probably of more use than the U-tube video.

• Pop-up blocker settings, exceptions and troubleshooting

The real question is what causes the ads.