See this - http://kb.mozillazine.org/Popups_not_blocked

I originally noticed this pop up after visiting a shopping site I've been to many times before "HolyClothing.com". It came up as "NetAxle" and "LivingPlay". I ran Nortons, and Malwarebytes Anti-Malware, both in safemode and nothing was found and the problem persisted even though my pop-up app is on. I finally added adblock plus and haven't had an issue since...at least so far. https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/