Just devil's advocate... Besides costs of running servers, also conceivable is potential competition with another service that they're trying to push (though they should have a smooth path to turn those users into uptake of the other service). Also conceivable is that there were costs for dealing with abuse of the service (e.g., excessively heavy users, human resources to respond to reports of criminal/harassing content). Or maybe privacy&security experts are finally shaking things up, and saying that Mozilla is still thinking too much like a dotcom.
Depending on how many users used the service, I'd say it could definitely cost a lot. It depends a lot on whether they're using something like aws/gcp where you pay for all bandwidth too.