Well, it depends... in Eastern Europe you could probably hire 80 developers with that money, while in California 15-20. "A few dozen" seems like a fair approximation.
If they hired remote it's not a crazy estimate..
Perhaps you should look up how much Mozilla engineers get paid. You can hire a lot of P2 engineers (including the often overlooked extra cost for the employer to cover insurance, rrsp, 401k, etc) for six million united states dollars. Especially if your company already works highly distributed.
That might be great if Mozilla developed some basic CRUD application. I highly doubt that what they need is a couple dozen more junior people (we'll overlook the fact that we are all ignoring what the word few means). If they actually had a strategy for the future, they'd probably need more P3s, 4s, and 5s. P5s are far more expensive. You also have equipment costs. Unless things have changed, they pay for a lot of home equipment including chairs. They also had annual get togethers that were extremely expensive per person and they frequently flew people around to pair.