Firebug displays js errors normally, up to the point where a call to Ext.Ajax.request returns (i.e. to a callback function).
The callback function executes normally, except that if there are any js errors in it, it fails silently without anything from Firebug.
I have been using ExtJS with Firebug for over a year .. and this is the first for this one ... mysterious .. and frustrating.
I haven't yet found a good way to find where the error is occurring other than to use the debugger and step through the code until it seems to go off track.