It's the same logic by which NGOs send physical mail and even small gifts like greetings cards and personalized stationery encouraging people to donate again:

That's not what I'm donating to an NGO for, but if doing so nets them more donations usable for their causes in the end, that's something I can get behind.

If Mozilla can find ways to generate additional income that also align with their values and don't put them into conflicts of interest, I can get behind that. (The management isn't exactly known for frugality and excellence in resource allocation though, so I'm taking it with a grain of salt.)