At which point the entry would still be cached by the OS, and since firefox apparently uses the OS API to resolve hostnames (if it didn't, I wouldn't have found the entry in my cache after opening the site, right?), it would still return the cached result (as long as it's not expired).
Besides, the question was how to prevent VAC from uploading your (hashed) DNS cache, and clearing Firefox cache doesn't flush those entries from your OS DNS cache.
