When you turn on Firebug, there's tab called 'Network' - there are shown all requests, especially ajax. The network tab allows you to filter requests and check request/response headers, status code, and message.
So, if you turn on Firebug and open Network tab (it's possible you'll have to enable tracking first, in that case it will show appropriate message), you will see if there was a request, where you expected it should be.
To see only ajax requests, check XHR on top bar, just under general tabs.
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