Presumably you aren't visiting that site with Firefox ?
If not (and I suspect so), it could be a website that you are visiting, regularly (which has a 3rd party link to that script) ?
If not there is a possibility that you have a malicious add-on installed in Firefox, check firefox Add-ons and check for new/unknown add-ons.
If nothing new, then I advise you to read this topic (
Logs to assist in cleaning malware) and follow the instructions and create a new topic in the Viruses & Worms forum (not in that topic) and attach the logs there.