Firefox show broken images

I know this question was asked a while ago but I wrote a blog post on this issue a while back and thought it might be worth adding a link in case anyone falls across it.

Edit 09/12/2013:

KatieK made a good point below about adding the information directly in the answer, here are the required steps to get this working, still relevant in the latest nightly build as of Dec 2013 (v28a2)

1) type about:config into your address bar and accept the warning

2) search for browser.display.show_image_placeholders and set to true

3) search your Fx profile folder for UserContent-example.css and rename it to UserContent.css (if the example file doesnt exists you can just create it)

4) add the following css to your UserContent.css and restart Firefox

/* Enable image placeholders */
@-moz-document url-prefix(http), url-prefix(file) {