I also use Firefox 3.6.6 but I use AVG Anti-Virus software and routine check with Adaware, Spybot, Spyware Doctor, Malware Bytes Anti-Malware and Super Anti-Spyware. I log onto the same site/page as you guys do and I never experience a problem.
I don't know if Len has read the latest in this thread but I'll make sure he sees it. Maybe he has some more input.
We have over a thousand visitors a day and it sounds like only two are experiencing a problem. That's the part that has me stumped, but anything is possible on the web. We'll keep digging.