User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:2.0b2pre) Gecko/20100629 Minefield/4.0b2pre (.NET CLR 3.5.30729) Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:2.0b2pre) Gecko/20100629 Minefield/4.0b2pre (.NET CLR 3.5.30729) Since I encounter this problem since a few days, this bug looks quite recent. When one use XMLHttpRequest (in asynchronous mode, with no particular setting), the readystatechange event is not fired anymore. This is of course quite problematic, to say the least. Here is a quick and dirty test case (sorry, I'm not able to host it somewhere at the moment): var xhr = new XMLHttpRequest; xhr.open('GET', location.href); alert('initial state: ' + xhr.readyState); xhr.onreadystatechange = function(e) { alert('current state: ' + xhr.readyState); } xhr.send(null); setTimeout(function() { alert('final state: ' + xhr.readyState);}, 1000 ); Reproducible: Always Steps to Reproduce: 1. Run the test case in Minefield Actual Results: The HTTP request is successful and the headers and the content of the page are correctly sent (you can see that with Firebug, for example). But the readystatechange event never happens. Interestingly, the readyState does change. The 'initial state' alert reports a state 1 (opened) and the final state reports a state 4 (done). Expected Results: Two alerts + one for each change of the readyState.

Works fine for me with a Jul 1 nightly build on Mac...

Surprisingly, the problem disappears when I disable the console panel in Firebug... So, I see two hypothesis: 1) Firebug doesn't play totally well with Minefield / 4.0 beta 1 or 2) Something in my Firefox user profile is broken But it seems this problem isn't related to Gecko itself, after all. Sorry for the useless bug...

Sounds like this Firebug bug: http://code.google.com/p/fbug/issues/detail?id=2927

Yep, indeed... John, do we have a Gecko-side bug tracking that issue already?

yes, one of the bugs that prevents firebug tests from succeeding on FF 4.0: https://bugzilla.mozilla.org/show_bug.cgi?id=557791 Bug 483672 regression: Security Manager vetoed action 557791 causes Show XMLHTTPRequests in the Firebug Console panel from working. I guess this is duplicate of bug 557791.

Sure looks like it.