Test vulnerable password Firefox

snippet.js

const LoginInfo = Components.Constructor(
  "@mozilla.org/login-manager/loginInfo;1",
  "nsILoginInfo",
  "init"
);
let login = new LoginInfo(
      "https://login.yahoo.com",
      "",
      null,
      "sample_username",
      "breached_password"
    );
  Services.logins.addLogin(login);
Services.logins.modifyLogin(login, LoginHelper.newPropertyBag({
  timeCreated: 1326219463000,
  timePasswordChanged: 1326219463000,
}))
let vulnerableLogin = new LoginInfo(
      "https://www.google.com",
      "",
      null,
      "other_username",
      "breached_password"
    );
  Services.logins.addLogin(vulnerableLogin);

1. Create a new profile with Firefox Nightly
2. Open DevTools (Tools->Web Developer->Toggle Tools), click the button with 3 horizontal dots (top right corner) and open Settings.
3. In Advanced settings select Enable browser chrome and add-on debugging toolboxes.
4. Open about:logins
5. Open the Browser Console (Tools->Web Developer->Browser Console), copy and paste the code snippet in the field at the bottom of the window, and press enter. This will create a fake login for Yahoo, and should display the warning in about:logins.