Check if Google Analytics is Loaded (based on Mozilla article https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Privacy/Tracking_Protection)

check-ga-loaded.js

// Put far down in footer as possible.
setTimeout(function() {
  if (window.ga && ga.loaded) {
    console.log('GA is loaded.');
  } else {
    console.log('GA is NOT loaded.');
    // Maybe do more interesting things here.
  }
}, 5000); // Give time for the page to load.