I have always found that Firefox does not necessarily show all thumbnails (I have used Ff since v 2.1).
I have found three work-arounds to combat this:
1)
Enable broken image placeholders in Firefox--just add a small addition to your css file like one of these two:
http://lab.gmtplusone.com/image-placeholder/
(The above link is broken, but I'll leave it here in case someone wishes to track it down somehow.)
http://megashare.altervista.org/blog...rs-in-firefox/
This shows you when
any thumbnails are missing.
2) If I suspect something missing I refresh (Ctrl+R) or fully refresh the page (Ctrl+Shift+R).
3) Ask the site programmer to optimize for non-IE browsers as well as IE.
Did any of this help?
e.d.