The footer of the results list says “Breach data provided by Have I Been Pwned” and it looks like Have I Been Pwned has an API here https://haveibeenpwned.com/API/v2.
I personally don’t see a benefit to Firefox Monitor, aside from a new channel of exposure and branding for Firefox, if they are providing the same data Have I Been Pwned is.
Trust. Mozilla is a relatively well-known trustworthy entity. "Have I been Pwned" sounds like some shady website that will steal your data. I've certainly never heard of it before, and Random Randy definitely won't have.
I personally don’t see a benefit to Firefox Monitor, aside from a new channel of exposure and branding for Firefox, if they are providing the same data Have I Been Pwned is.