Firefox doesn’t support canvas globalCompositeOperation darker. As it turns out, the darker composite value was removed from the canvas spec in 2007. See the globalCompositeOperation examples on MDN.

The closest solution is to use difference (which oddly enough isn’t in the spec), which Firefox (currently v28) does support. But difference is different from darker. difference subtracts channel values, darker multiplies them. If you’re using primary colors (#f00, #f0f, etc.) it may work.

IE10 supports neither.

See the Pen darker/difference canvas composite by David DeSandro (@desandro) on CodePen.