Ok this will be a small entry. I got this problem in a Drupal7 Web Page using a slider module, the classic rotating image block.
It was kinda strange, because even in Internet Explorer the images were ok (IE!). So after trying the same webpage in Opera and Safari, and seeing that it worked ok, I disabled my Firebug extension and it worked like a charm!. I enable it, and the problem was gone.
So next time you have this problem, a corrupt image in firefox only, maybe you should deactivate any Firefox plugin youre having right now, specially Firebug. Hope it help someone.