Hello,
I don't have FireFox, but I would suggest that you double-check your settings/options in FireFox to be sure that images are allowed to display correctly. Perhaps check the settings/options with any pop-up blocker you have (I don't know if that would have any influence, or not, but it is worth checking!).
Hope you find a solution soon, I can only imagine the frustration of not being able to see what images are in a listing!