When you get a message from Firefox Monitor, it will tell you the source of the data breach. That might be a site where you had an account, or it might be a data broker that sells people's contact information, or it might be someone that bought such information. If it's not a site where you directly had an account, they probably didn't leak your password or security questions/answers associated with your email address -- how would they have gotten them? Still, it's worth reading more about the breach if you have time. There may be additional information on the Monitor site as well as from Mozilla's data partner, https://haveibeenpwned.com/ .