AnonAddy and SimpleLogin also offer this, it's very neat. Added bonus that, assuming each email is service_name@domain.com, you'll know exactly who has leaked your email if you start getting random spam.
(Which is a shameless plug, since I work on that, but I know there are quite a few people here who trust Mozilla and for whom it's relevant.)
And if you use it to generate email masks in your browser, it can remember what website you created on without exposing that to the sender, and without a trail linking the email address to your other addresses.
This was all before Apple's feature came to be. I just hope this kind of email 'protection' becomes more mainstream.