Mozilla plans to extend the functionality of Firefox Monitor by displaying breach alerts to users of the browser.
The organization ran a Shield Study back in 2018 to test Firefox Monitor in the desktop version of the browser. The feature was passive at that time; users could check whether an account -- email address -- was found on hacked passwords lists, and they could sign up to receive alerts when a particular account was discovered on new leaked lists.
Firefox Monitor uses the Have I Been Pwned service but implemented the feature in a way that the full email address is never shared with third-parties.
Mozilla started to work on a breach warning system in Firefox in 2017. If things go as planned, Firefox 67 may be the first stable version of the Firefox web browser to warn users when they visit recently hacked websites.