IE 9 not works with xmlhttprequest in IBM Content Navigator

I am working on customization of IBM Content Navigator.

Following Java script code is used to create xmlhttpRequest.

	this.xmlHttpRequest = xhr._xhrObj ? xhr._xhrObj() : new XMLHttpRequest();

in IE version 9..xmlhttprequest is always null.

but in Firefox Version 32.0 i am getting xmlhttprequest value correctly.

why these difference exists in IE9 and FF 32.0?