window.onload = function () {
       ...
       CarObject.addCallback("StringEvent", function(arg1, arg2){
         alert(arg2);
       });
       ...
     }

Here's my js code: window.onload = function () { ... CarObject.addCallback("StringEvent", function(arg1, arg2){ alert(arg2); }); ... } CarObject is a js variable, it maps to a C++ object internally, here I add a callback to the CarObject. as you see, the callback function is not a global function, so the callback function will be freed by GC. We use JS_AddValueRoot() which will add the callback function to GC's root set, this means the callback function(JSFunction *) will be protected from GC. but I found that it caught GC won't work, many object will be leaked when firefox exit.