Firebase, GMS (Google Mobile Services). The Alphabet Corporation is part of many security and privacy conscious users' threat model, and these users aren't generally thrilled about leaking even limited message metadata like timing to their adversary, particularly when that adversary is known to cooperate with global passive adversaries.

There are actually two builds of Molly: Molly and Molly-FOSS. IIRC Molly uses regular Firebase, which can be faster and more reliable but comes with the above tradeoffs, while Molly-FOSS uses UnifiedPush.

Your point about exercising caution with forks of encrypted messaging apps is a great rule of thumb, and in general, social proof should NOT substitute for competent software security specialists reading and evaluating source code, but given you seem to trust GrapheneOS, it's worth noting that they've formally endorsed Molly: https://xcancel.com/GrapheneOS/status/1769277147569443309