firefox window.onerror event problem - alerts "Script error!" only

Question

I use the window.onerror to alert javascript errors for debugging.

window.onerror = function(msg, url, line) {
    alert(msg + '\nLine: ' + line);
};

When an error is fired, it can alert this actual error message in IE. But in firefox, it just alerts "Script error!", but I can still see the actual error message in firefox's error console.

I remembered several months ago when I worked on another project, firefox did not work like this. But I cannot get the code of that project currently. So I wonder what are the possible problems with this?

Answer 1

window.onerror(function(msg, url, line) {

You mean:

window.onerror= function(msg, url, line) {

Answer 2

I know the answer now. After I uploaded the file to a webserver (localhost actually), visited it through http://localhost/path/to/the/file.html, the window.onerror event works as expected (as in IE, alerting the actual error message instead of just "Script error"). But it does not work when visiting the file locally through file:///D:/path/to/the/file.html

Cannot figure out why?