Unexplained behavior across multiple sites could indicate an unwanted add-on. Could you review your extensions and disable everything nonessential (that's usually almost everything), restart Firefox, and see whether that shuts down the ads?

orange Firefox button or classic Tools menu > Add-ons > Extensions category

With respect to spyware or malware, I assume you have good real-time protection already. You might still want to run some supplemental scans. These two free tools are highly regarded:

Malwarebytes Anti-malware : http://www.malwarebytes.org/products/malwarebytes_free

SUPERAntiSpyware : http://www.superantispyware.com/

Thank you so much for your advice. I did exactly what you recommended and used an anti-malware program. The problem was immediately gone. Thanks again!