Last year, they promised to create an add-on that triggered when you visited sites known to have been breached in the past, and let the user check if his password was included in the leaked data, via HIBP: https://www.bleepingcomputer.com/news/security/firefox-will-...
Now, they announced Firefox Monitor, which is nothing but a standalone website where you can check your email and see if it's been included in public breaches. This is the same functionality of the main HIBP website. If people want to check if their email was included in a breach, they'd just visit HIBP, not Firefox Monitor.
Why does this website exist in the first place? They took a good idea that used a proactive approach to alerting users of potentially leaked passwords and they've created a Firefox-branded HIBP clone website that very few people are gonna know about or even use.
Pointless use of resources, when they could have used them for something actually useless.