I tested out the capx in FF, Chrome, NodeWebkit, and IE.
I seem to get the same additive blending effect in FF.
Below is an image showing screenshot comparisons of the four platforms I tested.
I changed the background image to a stone texture with a sharply defined edge, to make the FireFox blending behavior easier to see.
At the far right is a recreation of the FF example composed entirely in Photoshop, with the Logo set to additive (Linear Dodge) blend, and overlapping the stone. I lined up the recreation to match the FF example, down to the pixel, and the resulting Photoshop recreation is exactly identical to the FF version.
So it looks like FF really is using the additive blending mode for some reason.
Chrome: v40.0.2214.93 m
Hope this helps out.