firefox account code to check if a user is logged in, used within the browser

fxa.js

Cu.import("resource://gre/modules/FxAccounts.jsm");

fxAccounts.getSignedInUser().then((data) => {
  if (data !== null && data.email) {
    console.log('logged in with email', data.email);
    // logged in
  } else {
    // not logged in
  }
});