Pop Up Windows are showing up on my Mac

I have had my Mac for close to a year, and until recently I have not have any problems with pop up windows like I did when it came to my PC. Now that has changed. While there are not a lot of them, I still have to close out of at least 5 a night when I shut down my computer at night. Is there any spyware or any action I can take to stop this. What is most strange about this is I have my pop up blockers up when using firefox. At the same time, I still use safari at times because sometimes either one of these browsers in some way misbehaves. Does anyone have any idea what I can do to stop these pop up windows? Any help would be appreciated....

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