I will be on youtube or facebook ( I'm mostly only on these two sites ) and I will go to close firefox. When I am done, there is a full screen window that has popped up in the background. The ads are not relevant to the pages I am on. This just started happening recently, and it is very annoying to me. I've done a virus scan (avast) and also malware (SB:S&D) no viruses, no malware. Any help would be greatly appreciated.