It's not massively advertised on the homepage (though in some respects, I think outside of infosec circles "Firefox Monitor" probably sounds more professional/neutral than "Have I been pwned").
I thought the same when I initially was prompted about Firefox monitor at the bottom of Firefox's new tab page.
Was a little peeved at what seemed like a copy, but I have now realised it is just building on top of Troy's work [1] which is even better because of Firefox's larger reach.
They don't have to have a blinking marque text at the top attributing it to Have I Been Pwned. But they could have mentioned it on the front page somewhere that HIBP is one of their main sources. I trust HIBP, therefore, more value to Firefox Monitor had I known that link.
Though just realised, they're not that upfront about giving HIBP credit - If I were Troy this would peeve me a bit.