As I am getting caught up in the Firefox Builds forums, came across an odd (and not good) change to Firefox Aurora 11 (11.0a2) and Nightly 12 (12.0a1). A patch related to Bug 376997 makes it so that when an image is viewed by itself (either via right-click > View Image or putting the image URL in the address bar) the image will be centered on a black background. An example can be seen on </Glazblog>. Below is an example I created using Ken Saunders Mozilla Snow Globe image as viewed in Firefox 10 Beta and Firefox 11 Aurora.
Rob64Rock has suggested using the ImageTweak add-on to (somewhat, as it creates a new problem) fix this problem. With this add-on the image will be displayed centered on a neutral (grey) background or you can choose your own background color. However, since the ‘patch’ landed on Aurora the alignment is a bit off in Firefox 11 Aurora (11.0a2) and Nightly 12 (12.0a1), but does display correctly in Firefox 9 as well as Beta 10 (10.0b1).