Try opening a theming site in Firefox (gnome-look.org, for example), browse around looking at themes, and open them all in new tabs. After you get enough pages open with images simultaneously is when it starts happening for me. I tend to browse by reading through a few pages, opening all the interesting links with a middle-click, and then going through one by one after I finish the current page. I tend to end up with 40+ tabs at a time whenever I get into reading something serious, and if there's enough images (ads, screenshots, whatever), the corruption starts. Hope that helps reproduce it.
I happen to have a 4770 here so I'll see if I can reproduce on that, but honestly it's something we'd usually be happy with fixing as an update.