Redex-Firefox vs. Explorer
 

I noticed that while using firefox I don't get the red X's for broken picture links.
Just reporting I don't know if it is really a problem.

That's by design. I believe, from what I've read, that if you go to about:config and set browser.display.show_image_placeholders to false, you will get the broken image markers. Might have to restart Firefox to get that to work (it doesn't seem to be working for me right now, but I'm also using nightlies). Here's a broken image to test:
http://www.tfproject.org/tfp/images/...ttfproject.gif
The image should be above this line.

Nope, that's just to give a frame until the image is done loading