it's probably because of the way firefox displays it's pages in the loading process..
IE often times loads a few of the images first before it starts to display the page, it seems to be random, sometimes it'll display no images and load them one by one screen, or it'll load everything and then display them afterward..
Firefox and Mozilla appear to load the document first and then load all the images one by one.
That's probably why you never noticed it before. For your connection, IE seemingly loads everything at once with a few seconds of what it appears to be searching for the page.
Firefox will load the page seemingly instantaneously followed up by the images afterward.
Yarr join LoGD and defeat your fellow coders!