Till Firefox 11 when any image is opened in a new tab in Firefox, it was displayed on white background at top left corner of the window and even Chrome which Mozilla follows in implementing UI changes also offers the same behavior, however things changed after Firefox 11 release Firefox now displays the image on the dark background with picture position at the center. Do you want to view images on dark background or on white background?, if you prefer the latter you can get that behavior with Old Default Image Style add-on for Firefox.
Why Mozilla changed image view to dark background?
Some one filed a bug asking it will be better to render standalone images against a dark background at center position instead of white at the top left corner. Mozilla then added style sheet to top-level images which forces image background to black and position to the center.
Old Default Image Style add-on restores image viewing in Firefox to older style
There is no about:config hack to bring old image style to Firefox, but to fix this annoyance no choice you’ve to download and install Old Default Image Style add-on in AMO here. No restart and configuration is necessary, install it and you’ll view image at top left corner with white background.
From the extension Options you can even set custom background color and even center the image also to your liking if you want.