Firefox for sure has support for that since Firefox 1.0 so in terms of
Firefox you can be sure that any released version has support.
In terms of Gecko/Mozilla I have run a quick test with Mozilla Suite 1.0
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.0) Gecko/20020530
and it exposes a method named evaluate on XML DOM documents and
evaluating a simple XPath expression that way worked so the API is there
since Mozilla 1.0.