I don't know how this falls in line with their comment "Your email will not be stored."
No email address is transferred. To quote Mozilla:
When a user submits their email address to Firefox Monitor, it hashes the plaintext value and sends the first 6 characters to the HIBP API.
Thanks for that, it could be useful if they either stated that (or gave a link) on the same page as they say your email won't be stored.
Mine you those like myself, who question statements like that, are less likely to need the service. Those that believe just because they say it (and it is written down) is so may be their target audience.