There's a world of difference between an exploit being known to someone and that exploit being put up for sale on the black market. In either case, if the researcher who found the exploit sold it, that hardly makes Mozilla complicit in his actions.
I am not sure that being asked not to disclose is a good enough reason without further justification; in fact it seems like a poor reason to me. Mozilla is in my view kind of a shepherd for internet users and I'd hope they'd fall more on the side of "let's not let our users get owned unnecessarily" than that of "let's sit on this vulnerability just because the disclosing party asked us to."