No pages will open on Vista

Running from a reformatted cruzer micro U3 drive, on Vista Ultimate with UAC disabled, Windows firewall, Avast! antivirus. Firefox opens OK, but with an untitled tab. Clicking a bookmark or entering a URL results in exactly zero activity. FWIW FileZilla Portable opens and can resolve host names to IPs but can't contact any hosts. Both programs work fine on an XP machine. Disabling Windows firewall and/or the antivirus has no effect.

I presume it's some setting on my system, maybe having to do with running from flash drives, but what?