Oh yes, since FF 3 beta 4, the DOMinspector is not anymore distributed with FF. You need to install it as a separate extension.
Oh, XPather has always been just a small development-utility toy-project, though it helps (a lot?). Anyway, the facts are inspiring: 2,5 year old; 58000 downloads; very positive feedback; $7 donations (+5 empty promises); $27 adsense. The mission: contribution to public successful. Isn’t it? ;)