I spent some time debugging why progress events weren't working in an extension. It turns out that I was setting the onprogress handler handler after open() was called. When you do this, the handlers are not called at all and there's no exception or error raised. I'm not sure what the spec says about it, but if this is not allowed it would be nice to have at least some sort of feedback. Testcase: Start from: http://mozilla.pettay.fi/xhr_upload/xhr_upload_demo.html Put the line with xhr.open("POST", "cgi-bin/posthandler.pl"); before the lines setting the onprogress handlers.