They do that, with the extra step of allowing k-anonimity (https://www.troyhunt.com/were-baking-have-i-been-pwned-into-...)

That's... exactly what they're doing?

Double-checking source code @ haveibeenpwned.com. I see no javascript that hashes your email address before submission.

Sure, but that's not what the linked article is discussing as the basis for integration with Firefox.

Well the email k-anonymity is new that he added for this integration and Troy addresses why he’s not yet using it on HIBP in the article.