So, I understand that this is a string and can therefore, formally, be anything. But I'd guess that there's a much smaller set of possible values that are actually likely to be useful to users of this API (such as the two listed, and any more?) and listing them would make the documentation much more helpful. Also, this value maps onto https://developer.mozilla.org/en-US/docs/XUL/window#a-windowtype, via https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsIWindowMediator#getEnumerator%28%29, and mentioning that would help too.

Otherwise this documentation doesn't give me much idea of how to use the parameter.