Just because it said Mozilla Security in the title bar does not mean it is from Mozilla as Mozilla nor the Firefox browser has ever done such a thing. This sounds like one of those fake animated scanners which claims it is scanning your computer and then says you are infected with such and such. It then suggests to download something that will be what really infects your computer if using Windows. As long as you did not download and install this whatever.exe then you should be safe. Though you may want to scan your computer with whatever antivirus software you have with updated definitions just to be safer. This sort of thing is even more obvious on being fake when Linux and Mac users can encounter these pages.