Comment by daveguy

Firefox is using the wonderful haveibeenpwned resource. There is also a public API for haveibeenpwned if you want to incorporate it into your own clients:

https://haveibeenpwned.com/API/v2

Please note rate limits/ abuse policy so everyone can use:

https://haveibeenpwned.com/API/v2#RateLimiting

(I am not affiliated)