Embedded image is supported on Mozilla/Firefox. But please pay attention to the code in GreaseMonkey's user script, if the image data format violate the limitations on IEScripts, an error maybe occurred.
Limitations on IEScripts:
The embedded image format can only be used in user scripts
LF/CR is not allowed in the image data part
Escape char (\) is not allowed in the image data part
Only base64 encoding and the standard %xx hex encoding supported