Avast blocks this once a day... hxxp://s.vb3k.com/crossdomain.xml

I see other people are having the same issue, but in the Forum, the help seems to be specific to each machine. I’m not sure I can even get help here, because I’m not too computer savvy. I have run a bunch of scans, adware cleaner, malwarebytes, spybot, AVAST etc… BUT nothing is found… ???
NOTE: Infection location is Firefox – do not use chrome…

https://forum.avast.com/index.php?topic=164210.msg1170555#msg1170555