There used to be a Firefox extension called Scrapbook. I think it was one of the winners of one of the two first extension competitions.

It could easily download entire sites or just a subtree, including resources. It would the rewrite URLs as necessary to point to the downloaded documents instead of pointing to absolute URLs on tje Internet.

Still hoping that Mozilla will improve its extension API. I mean I can see the reason for not allowing everyone to pone around everywhere under the hood, but the current API is rather anemic it seems.