Firefox is affected by a bug that prevents from reading the responseXML property of XHR objects if the document.domain has been set.

See Bug 326337 at

This bug does not cause any issue to Lightstreamer. But if you perform your own XHR calls together with Lightstreamer (that sets the document.domain), and try to read the responseXML, you will see a security exception in Firefox.

You can find a workaround on the bug page. That workaround is declared working from FX and is reported here:
var doc = (new DOMParser()).parseFromString(req.responseText, "text/xml");
This bug was fixed on Firefox 3