One odd thing about the Firefox Send shutdown is that the security community repeatedly asked Mozilla to add a "Report abuse" button to help take down known malware links. Instead Mozilla shut down the whole service, which was a bit extreme. So I think abuse was, at least in part, a convenient cover story for the planned shutdowns/layoffs/refocusing that took place at Mozilla at the end of 2020.

We think it's encouraging that other products that offer end-to-end encryption like Signal and WhatsApp have managed to handle abuse, malware, and other threats.

IMO they shouldn't, that's the whole point of a privacy focused service like this - no one should be able to view/inspect what you're sharing on it.