As TFA says, it's about Mozilla's employees. "Thinking you know best" is an attitude that leads you to fire people for out-of-work disagreements, and also remove features that "no one uses", and also divert money from useful products (Firefox) to products that they are "just sure" will be a huge hit (Firefox OS) but then turn out to be total flops.
Mozilla's problem is that its organization (the collective outcome of its employees) is extremely arrogant. Arrogant about technical choices, arrogant about product design choices, arrogant about the way they do PR (including shaming a developer after he died), and arrogant about political/ethical choices as well. That's not to say they aren't ever right, but an organization that approached political topics where Americans are evenly divided with a bit of humility might have an easier time hiring just as an organization that approached product design topic where it's users are evenly divided with a bit of humility might have an easier time finding a good design. Humility helps, and Mozilla has none of it.