K,
I have a Javascript working in IE6, but works in FF2.0.0.4 for a short while than hangs.
(Win XP)

Here's how to reproduce:
http://www.outofthebluesolutions.co.uk/tm/t.asp
(also opens a popup for my own debug data)

(1)On page open, the request calls to NDR.asp, returns "Art" and populates the navigational tree.
(2) Click on the '+' next to ART, and the request is called...