rem: const disabled = gBrowser.browsers.length == 1; 
add: const disabled = (!gBrowser || gBrowser.browsers.length == 1); 
context: function updateGroupmarkCommand(){  const disabled = gBrowser.browsers.length == 1;  document.getElementById("Browser:AddGroupmarkAs")          .setAttribute("disabled", disabled);}
meta: 5872	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/5872/dec6c92b11dd1101f5886210c4b3d1f4c118c71a/navigator.js/buggy/suite/browser/navigator.js