Node.baseURIObject - Web APIs
the node.baseuriobject property returns the nsiuri representing the node's (typically a document or an element) base url.
... syntax uriobj = node.baseuriobject value an nsiuri.

nsIDOMParser
(this section is only relevant to firefox extensions--not to web content.) to create a document, the parser needs to specify a principal (see security check basics), a base uri (see document.baseuriobject), and a documenturi.

Index - Web APIs
2702 node.baseuriobject api, dom, dom 3, dom reference, gecko, node, non-standard, property, read-only, reference the node.baseuriobject property returns the nsiuri representing the node's (typically a document or an element) base url.

Node - Web APIs
node.baseuriobject read only (not available to web content.) the nsiuri object representing the base uri for the element.