My customer who had been having this problem with FF3.6.3 had been on the website just before I did the update and came back soon after I put it back online.
So.... taking a stab here I think it could be a "caching" problem with FF caching some of the pre update files and causing confusion with post update files.
Customers who came "clean" to my site have not had any problems. The people not having problems may not have any of the pre update files cached in their browser.
I suspect this problem will just go away after a few days as the caches are refreshed.
ISP's also cache some files so clearing the browsers cache alone may not immediately solve the problem
PS: My customer came back today with IE8 and completed her order

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