I think you would need to create a custom style rule for this change. I don't know the most selective approach, but this works and you could watch for side effects.

(1) Install the Stylish extension: https://addons.mozilla.org/firefox/addon/stylish/

(2) After restarting Firefox, click the Stylish "S" icon on the Add-on bar > Write new style > Blank Style

(If you do not usually display the Add-on bar, press Ctrl+/ to display it. If the "S" icon is not there, drag it from the Customize dialog, which you can show using Alt+v > Toolbars > Customize)

(3) In the editor, paste this:

@namespace url(http://www.w3.org/1999/xhtml);
body > img.decoded{
  background-color: transparent !important;
}

(4) Give it a name, and click Save.

The OP is right, there has got to be a way to change this is the about:config module.

I want to remove the white background also, and I don't want to install and add-on just to do that.

Can we get some pros on this asap please :D

Hi greyhood, you can add custom style rules to userContent.css and userChrome.css if you don't want to use the Stylish add-on.

I will let a pro explain how to do it. ;-)

haha i knew saying the word pro was going to stir the pot a little bit, that's what i said it :D

anyway, i appreciate your support for the alternative method, just waiting for a non-add-on fix

You can also experiment with code like this in the userContent.css file.

• http://kb.mozillazine.org/userContent.css

html > body > img:only-child {
 background-color: #FFF !important;
 box-shadow: none !important;
 margin: auto !important;
}

Thanks for your feedback. I made a restartless add-on that will revert the behavior of standalone images to be rendered against the grey background.

https://addons.mozilla.org/en-US/firefox/addon/transparent-standalone-imag/

You can install this add-on and then it will all happy time :)