I see a comment below from a Mozilla employee stating that "there is limited public information available about this deal", and claiming that the best quote is in an article that contains a paraphrase from an email from another Mozilla employee saying that Pocket didn't pay for placement.

When a non-profit gives prominent placement to a for-profit company, there are many ways for money to change hands beyond straight up payment for placement. I think we are owed a clear explanation of the Mozilla-Pocket deal.