And what happens when FireFox decides to drop this option 1-2 years into the future? I reckon they'll give time to change the email address on all the pages one used it for, but still...

nvm, it's in the FAQ:

"What happens if Mozilla shuts down the Firefox Relay service?

We will give you advance notice that you need to change the email address of any accounts that are using Relay aliases."

Note that one cannot reply using this service (yet). So the whole anonymity is gone as soon as one wants to contact some service without disclosing the real address (?)