Well, I tried, but the thing is that those guys at Mozilla could do it better. They have the test, the picture and yet another 2x times performance improvement, awright.
But lets check it out by ourselves, shall we?
Seems like RightJS implementation murks a native feature again. It's more than three times faster on classes adding :)
So how did those guys at Mozilla get their results?
They simply used lousy RegExp based implementation of the feature in their test. Surely it was slower.
This is pretty much it. Hope they will fix it in the next releases of FF.