Probably because "you have to support it". If it does not work out of the box, but does work in Chrome, noone is going to bother to fix it. Same thing happened to IE - required additional code in order to work, and devs ignored it unless requested to do so.
So if FF made sure that everything is matching 1:1 (css, js), then it should just work... But sometimes as each browser decides to implement same thing using different naming.. Things break.
So if FF made sure that everything is matching 1:1 (css, js), then it should just work... But sometimes as each browser decides to implement same thing using different naming.. Things break.