timeout issues with XMLHttp object under some versions of firefox

Hi.
I am using the XMLHttp object to make an asynchronous request to a php
script.
The idea is that the script does not respond immediately but only when new
data is available. The connection is left open with nothing being sent on
it.
It works fine with opera and konqueror even with delays of about a minute,
but fails with some version of firefox (1.0.7 linux at least): the
onreadystatechange callback function is never called when the delay is
above around 10 seconds, although ethereal shows me that the data is sent
by the server.

Is this a known bug? Is there at least a way to be informed that a timeout
occured?