Open source means the source code is available in full. Whether the creators/inner contributors run the project like the wild old days of Wikipedia or whether they are autocratic is up to them. e.g. I can't just tell Linus to put my code in the Linux kernel.
But if I have the motivation I can take the source and put in my changes myself. Perhaps it will gain traction, perhaps not. But I have the option.
Also, there is the little matter of transparency which only open source provides.
So in these ways Firefox is the only one of the aforementioned which is truly open, even though, as you say, they do not allow you any say in the development of their OS.