Recent versions of Firefox have a much improved Browser Console (formerly known as the Error Console, and before that known as the JavaScript Console). It's lik

Recent versions of Firefox have a much improved Browser Console (formerly known as the Error Console, and before that known as the JavaScript Console). It's like a browser-wide version of the per-page Web Console that showed up about a year ago. It shows HTTP requests (among many other things), and may be quite sufficient for diagnosing this problem. Press Ctrl+Shift+J to open it, or click Firefox -> Web Developer -> Browser Console.