If you want to provide “features” commercially, it’s much easier to just use Chrome’s engine. The point of Firefox, at this time, is that we have at least one alternative implementation.

MailMate is closed source donationware with a bus factor of 1, which is just hilarious. As I understand it, the “sell a mail client for a one-time payment” model turned out to be insufficient to support the single developer, leading to the whole “patron of a for-profit company” theme.