The premium paid subscription is said to be only available in specific countries, but the payment form seems to appear in other places too. So I’m not sure how the service allows or disallows subscriptions.
A quick thought also occurred to me comparing this with iCloud email aliases from Apple, which is available for all paid iCloud subscriptions starting at the same price as this one ($0.99 per month) and allows the user to use their custom domain (Firefox relay premium gives you one custom subdomain under mozmail.com). And for the same price, Apple also provides 50GB of storage and supports the iCloud Relay hop service for Safari (and apps, if supported).
I’d like to support Firefox monetarily, assuming the revenue from this service goes to Mozilla Corporation (not Mozilla Foundation) and to Firefox. But the attachment size limit is currently unacceptable for me.