Just found something interesting while researching a problem Dianne was having with a website SRT was working on. In checking various browsers throughout the office (FireFox, IE, Opera), a page looked ok on most of the browsers, but not all. In fact, there were just a couple of machines running Firefox that had problems (others, running the same version of Firefox had no problem viewing the page correctly).
Then Dianne asked me if I was running Adblock Plus (which, of course, I was). This was the key. Once we realized some of our images were being blocked by Adblock Plus, I checked the image as well as Adblock's filter rules. It turns out the images were placed in a folder on the website called "ads". Adblock Plus, by default, has a rule to block images found in a folder called "ads" (with some exceptions):
I disabled the rule to verify my hunch. Once disabled, the page displayed correctly. So, be careful where you place your images on your website!