What is your home page?

I would try doing two things first:

Reset Firefox Refresh Firefox - reset add-ons and settings

If that doesn't help, then try reading Troubleshoot Firefox issues caused by malware

I read more on this after posting my problem. My home page is MSN.com. This is probably a site that Microsoft Advertising automatically shows "personalized ads". You have to manually close the ad, or follow the instructions from Microsoft for opting out. However, you have to opt out on each computer you use. Also, if you periodically delete all of the cookies on your computer, you have to find the Microsoft site to opt out again.

Or just install something along the lines of ad-block plus: adblockplus.org