Apologies for being slightly off-topic, but I'd argue this would be questionable even if it were an openly announced partnership like with Yahoo. Yes, Mozilla is free software and is doing some enormous contributions to the open web. Which is why we should support them with donations and code contributions. (And I'd also fully endorse usage of paid services from them should they develop any).

However, what they are doing now is basically selling their good reputation and their wide user base to force some features on their users that they don't need in return of (presumably) payment. That strikes me as deeply unethical.