AFAIK Monitor does not have this feature, but the service it uses in the background, has this. Take a look here: https://haveibeenpwned.com/DomainSearch

Any plans to add this feature in the future?

I have this issue too as I own a domain and use meaningful account names.

So, for instance, I could have google@example.com, mozilla@example.com, netflix@example.com, amazon@example.com etc.

This meanst that any time I want to find e-mail from a particular company, I just search on their name and they all appear.

It also means that if I start getting spam addressed to company@example.com, I know who has been passing on my details to others.

But I can't (even if I could remember them) put every single meaningful account name into Firefox Monitor, so I would find it very useful to be able to find any compromises of *@example.com