Marketplace product pages cannot be viewed in Firefox 5. It appears that this is due to some tracking pixel you are using:

http://tracking.searchmarketing.com/welcome.asp?SMCID=1982&x=

What happens is the page loads briefly, then it refreshes to a totally blank screen and the browser sits there trying to load this thing. It does not alter the URL in the address bar like a page refresh, but if I use Firebug to "Inspect Element" on the blank page is shows that it is because this tracker has failed to load. It does not do it anywhere else on the site that I could find, only actual product pages. It is possible that their server sends incorrect/non standard headers that Firefox will no longer tolerate or something but I really don't know.

What I do know is that the one thing I can no longer on Renderosity is buy something. I'm not inclined to change browsers to accomodate a third party attempt to track me.