display broken images in firefox

Otherwise, we won't know if there's a broken image in a web page.

A quick tip on enabling broken image placeholders in Firefox 3 and beyond

In short, put the following in:
Firefox/Profiles/[depends]/chrome/userContent.css

/*
 * Show image placeholders
 */
@-moz-document url-prefix(http), url-prefix(file) {
  img:-moz-broken{
    -moz-force-broken-image-icon:1;
    width:24px;
    height:24px;
  }
}