hello Labeliteteta, you can use the following addon as a workaround: https://addons.mozilla.org/firefox/addon/transparent-standalone-imag/

Thanks, but why it is don't default?

it has been the default for a short time but apparently it was reverted in firefox 15, because there was lots of user feedback that the rendering of transparent images on dark background would be troublesome (though i don't know anything about the details of this and the cases where such issues would arise, i'm merely referencing this blog entry by a mozilla dev).

You have a problem with images that have white details and do not display well on a light (white) background.

Other images may have dark details (e.g. line drawings) that do not display well on a dark background, so there is always a trade off. It would be good if you could right-click such a standalone image and set a specific background color if the default isn't working.

• bug 754133 - Set background of standalone images (the image itself, not the whole page) to white

(please do not comment in bug reports)

It is added by this stylesheet:

• chrome://global/skin/TopLevelImageDocument.css
• resource://gre/res/TopLevelImageDocument.css