Save a WHOLE site locally to your hard disk? Not since IE for Mac & probably never again. I've used the Screengrab! extension with Firefox 3.0.x and it works AWESOME! It is THE reason I switched to Firefox from Safari.
Only saves one page at a time, but the copies are PERFECT! Will not save automatically however, but I don't think anything will(?) If you need to save webpages to exist after the original site is long gone, then Screengrab! is the tool! (for the Mac anyway)
[Note: currently NOT compatible with Firefox 3.5.x]
HTH!
Russell