I can download the Firefox sources and everything else they produce.
That they make money incidentally to that is really no problem and a positive because it provides reasonable funding.
What if Firefox made a world beating browser by accident. Would they be justified in closing the source, restricting access and making people pay for it?
Anyway, to answer your question, no, not okay to close up the nonprofit and go 100% for-profit in that case.
Concisely, in any human matteres:
Do what you say you'll do, or, add qualifiers/don't say it.
Take funds from a subset of users who need support services or patch guarantees of some kind, use that to pay people to continue to maintain and improve the product.
They had one of the best browsers in the world at one point.
Their sell-out path was hundreds of millions of dollars from GOOG to make their search engine the default, and, unspoken: allow FF to become an ugly, insecure, red-headed stepchild when compared to Chrome.
Likely part of what took priority away from Thunderbird, at the time, too.
That they make money incidentally to that is really no problem and a positive because it provides reasonable funding.
What if Firefox made a world beating browser by accident. Would they be justified in closing the source, restricting access and making people pay for it?
That's what OpenAI did.