Mozilla announced a new light weight skins for your browsers. You can choose among thousand of skins given according to your mood. Here the dilemma in Mozilla’s mind clearly reflex because when Mozilla first  introduced a  skin in 2007 because Mozilla was previously focussing on a fast speed browser speed

and now by using these custom themes your browser probably get slight slower. But Mozilla know about the need of the users so it takes this innovative step because many users need look rather than speed.