Some URLs give away their affiliation but that one is inscrutable to me... A few possibilities come to mind: (1) A site you are visiting runs (or has been infected with) that ad (2) One of your extensions loads the ad (3) [if it comes up in a new window] Other software on your computer launches the ad in your default browser (4) Other software on your computer intercepts/modifies your browsing (5) Your ISP injects the ad into pages As a starting point, you can supplement your Norton security software with the <strong>scanning/cleaning tools</strong> listed in our support article: [[Troubleshoot Firefox issues caused by malware]]. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.