I've been working at Mozilla for a few years now, although this is my first job I'd say this is really great already. One Venn diagram I saw in the past was about choosing your job and the title read: pick two among "interesting", "well-paid" and "ethical". Well, I think you can easily get all three when working at Mozilla, or at least that's my experience of it.
Also, I'd suggest that some people have very high expectations when they start at Mozilla, hoping it works like a perfect anarchist (in the actual political sense) and democratic organization. It's not the case: Mozilla Corporation is a company (duh!). So it's slightly less corporate than the regular public company (e.g. there are no shareholders), but it is still working like a regular company: vertical hierarchy, meritocratic do-ocracy, OKR, you name it. Some people don't like that and tend to be more virulent towards Mozilla than they would be towards any other corporation, hence the bad opinions on Glassdoor, in my opinion.