Windows 7 64-bit
Brand new profile
Making a request to a straight up image (ie: http://192.168.56.101/image.jpg), Firefox requests the image twice.
Initially, when I discovered this, I was running through an internal development environment and was able to see the webserver get the request twice. I setup wireshark and confirmed that FF was indeed requesting the image twice.
I setup IE 8 and the latest Chrome and confirmed that neither of them make a double request.
I then installed the lastest HTTPFox and made a request to an image outside of my control (http://g-ecx.images-amazon.com/images/G ... 78229_.jpg) and was able to see the image get requested twice.