Firefox 1.0 is affected by a bug that prevents to issue any XMLHttpRequest' afrter the document.domain has been set, even to the original domain.

See Bug 290100 at

This bug does not casue any issue to Lightstreamer. But if you perform your own XHR calls together with Lightstreamer (that sets the document.domain), you will see a security exception in Firefox 1.0.

You can find a workaround at

This bug has been fixed in Firefox 1.5.