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: 9421	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/9421/e9c7c1a8bc95ae0846cb122f33a9de18124f7c78/navigator.js/clean/suite/browser/navigator.js