Error: permission denied to get XULElement.selectedIndex

[Paul Thompson]

I am getting an odd, inscrutable error in Mozilla Firefox. When I use
an array to shift focus to an element, I get the error

Error: [Exception... "'Permission denied to get property
XULElement.selectedIndex' when calling method:
[nsIAutoCompletePopup::selectedIndex]" nsresult: "0x8057001e
(NS_ERROR_XPC_JS_THREW_STRING)" location: "JS frame ::
http://www.biostat.wustl.edu/~consult/demo/java/mainjava.js :: nclkH ::
line 292" data: no]
Source File: http://www.biostat.wustl.edu/~consult/demo/java/mainjava.js
Line: 292

However, the focus() works fine.

1) How do I interpret this error?

2) Is there some error dictionary or something for FireFox?