Exception: Unable to get jsdIDebuggerService.flags in FF 4.0?

Hello,

I would greatly appreciate if anyone here could help me.

Normally, the jsdIDebuggerService should have a "flags" attribute.
Developers can either get it or set it. And I have been successfully
doing so prior to 3.6.

Roughly, my code looks like:
var jsdService = Cc["@mozilla.org/js/jsd/debugger-service;
1"].getService(Ci.jsdIDebuggerService);
jsdService.asyncOn(........);
jsdService.flags |= Ci.jsdIDebuggerService.COLLECT_PROFILE_DATA;
......

Although it works in 3.6 or earlier versions, the last line of the
code above doesn't work in FF 4.0.

What it says is a "NOT_AVAILABLE" exception:

[Exception... "Component returned failure code: 0x80040111
(NS_ERROR_NOT_AVAILABLE) [jsdIDebuggerService.flags]"
nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)"  ]

Could any one help me answer: How to set the debugging flags in FF
4.0?