(I work for Mozilla)
I'm guessing the poster is legitimately former staff. It's not hard to find disgruntled people in any organization (especially if you look at those who have left.) And they'll often have legitimate reasons.
But the question is really whether there's a consistent pattern of problems - actual rot, so to speak. I haven't seen much, but then i know that some parts of the org are very different than others. I can say that I have publicly complained about a number of things in the last several years, and never felt any repercussions as a result. That includes comments made directly to the CEO during All-Hands sessions, so I'm not just talking hypothetically.
Yet I have also heard about a handful of cases where people have been treated unfairly as a result of public comments or actions, including a couple of friends of mine. So shit happens here, it's definitely not perfect and the problems aren't all in the past. But overall, I still feel like Mozilla is substantially better than most similar companies.
Just my perspective.