Ex-Mozilla PM here, I completely understand what you mean by this and I generally agree. I'll go a step further and say that as a PM I often didn't have a choice, either, decisions were being made above me and I often found out from the engineers that they'd been told to do something different than what I'd just expended significant effort on documentation to support doing. A primary reason why I left is that I felt like my wings were clipped as PM, and that I was unable to effectively build symbiosis with engineering. I'm more technical than most PMs at Mozilla were and had a better relationship (I think) with engineering than most of the PMs did, but it was a fundamentally untenable situation to be in, where I ended up just being a middle-man, which is not what a PM is supposed to be, and it doesn't create good products or user experiences.
Somewhat ironically, I think that Mozilla needs a stronger Product organization to succeed, but that wasn't happening. If PMs are doing their job right, they are there to advocate for the users/customers and ensure that the direction of the product aligns with how people are using it. At Mozilla, it felt to me like there was a very heavy top-down approach and with some exceptions, most product features or projects were focused on enabling alternative revenue pathways without regard to how this alienates existing users. Very little of what I was asked to work on had any chance of moving the needle on market share, which was and is the fundamental issue for Mozilla existentially.
