code: function blockCmdArrowNavigation() {
          if (Env.mac) {
            editor.on('keydown', function (e) {
              if (VK.metaKeyPressed(e) && !e.shiftKey && (e.keyCode == 37 || e.keyCode == 39)) {
                e.preventDefault();
                editor.selection.getSel().modify('move', e.keyCode == 37 ? 'backward' : 'forward', 'lineboundary');
              }
            });
          }
        }
docs: Firefox on Mac OS will move the browser back to the previous page if you press CMD+Left arrow.
  You might then loose all your work so we need to block that behavior and replace it with our own.