Open the url or the minimized testcase using a debug build, and watch the console. for each frame of the animated gif, this will come: WARNING: Frame setting default background color but it has transparent background!, file ../../../../../mozilla/layout/html/base/src/nsFrame.cpp, line 4082 This is 100% reproducible, unless another copy of mozilla is running it seems.
Created attachment 63220 [details] minimum testcase, two tags setting the background imeage with CSS doesn't cause the warning, only setting it in the <body> tag does.
The testcase is broken. It does not work as a GIF. A possibly related bug, bug 12037, was recently fixed. Could you download a current nightly build and retry?
I am not seeing this, that message doesn't seem to be in the src any longer. Marking fixed.