People trust Apple and nobody else can read their code. This suggests open source is not necessary to build trust.

No, people choose comfort and projecting an image of status over privacy. It's not because they trust Apple.

I'm afraid Firefox is so abysmal noone could comprehend the whole project's code. Guess, we could only review a tiny bits of it (I had briefly read sync-related parts of code in hope I could replace them with something saner and simpler, but ditched the idea) and hope others did the same for other parts.