XMLHttpRequest upload progress event doesn't work in Firefox 10.0.2

问题

I'm trying to create an ajax based upload form with progress bar. This is my code (the upload part of it):

$xhr = new XMLHttpRequest();
            $xhr.upload.addEventListener("progress",
                function(e) {
                    if (e.lengthComputable) {
                        $progress = (e.loaded / e.total) * 100;
                        $('#file-progress').css('width', $progressWidth * (e.loaded / e.total));
                        $('#percentage').html($progress.toFixed(2) + '%');
                    } else {
                        alert('Y U NO WORK?');
                    }
                }
            , false); 


            $xhr.onreadystatechange = function(e){
                if($xhr.readyState == 4) {
                    //i'll add this later
                }
            };

            $xhr.open('POST', 'handler.php', true);
            var $data = new FormData();
            $data.append('file', $file.files[0]);
            $xhr.send($data);

Everything works in Chrome (the progress bar and file upload) but in Firefox works only file upload without the progress bar. No errors, nothing. Firefox ignores the progress listener and i can't get why because as i have read Firefox should support XMLHttpRequest level 2.

来源：https://stackoverflow.com/questions/9550940/xmlhttprequest-upload-progress-event-doesnt-work-in-firefox-10-0-2