I see this argument all the time in the form of "why are they working on the UI instead of the backend?" Well, because UI engineers and backend engineers are different people and don't share one-another's competencies, is why. The people who are adding these things probably don't know how to improve Firefox's performance. They're probably networked-multimedia engineers scratching their own itch.

(And it's not like Firefox's performance has any low-hanging fruit left; there have been years of performance improvements already, and only someone versed in those would know how to take them further.)