Check if Firefox is in private mode

isFirefoxInPrivateMode.js

var isInPrivateMode = () => {
  if (!window.indexedDB) {
    return Promise.resolve(false);
  } else {
    return new Promise((resolve, reject) => {
      const name = 'test';
      const DBOpenRequest = window.indexedDB.open(name);
      DBOpenRequest.onerror = () => resolve(true);
      DBOpenRequest.onsuccess = () => {
        const db = DBOpenRequest.result;
        db.close();
        const deleteRequest = window.indexedDB.deleteDatabase(name);
        deleteRequest.onerror = () => reject();
        deleteRequest.onsuccess = () => resolve(false);
      };
    }).catch(() => Promise.resolve(true));
  }
}

isInPrivateMode()
  .then((isInPrivateMode) => console.log(`Private mode: ${isInPrivateMode}`));