Firefox 15 renders standalone images created in file formats like GIF and PNG against white background, transparent images created with Photoshop and other image editing software will be rendered on same white background too which looks ugly this was not the case for versions before Firefox 15 where they have been rendered on dark grey background. Meet Transparent Standalone Images add-on which renders images with transparency against textured grey background so that you can see start and begin of transparency.
I came across this userscript at http://userscripts.org/scripts/show/153158 to show standalone image transparency. I found that it’s easier to use. You can even change the background color.