In this case, the decision is arrogantly wrong because it's predicated on an over-inflated sense of Firefox's position relative to the entire platform and ecosystem of consistent applications, and this position is based on self-interest, putting branding interests and their designer's ego ahead of the interests of their customers.
Design that doesn't put the user first is egotistical by nature.
There's nothing to get used to; design isn't new on Apple's platforms. If anything, Apple has driven software design more than any other platform or company.
Apple's customers value -- and paid for -- a consistent ecosystem, on which Apple defines design guidelines and HIG requirements. Apple developers and designers work together to maintain a consistent ecosystem and the resulting mutual benefit.
