Since you changed the FF rule to Allow access, did you stop and restart v3? (as in, Exit from the systray icon, then restart from desktop icon) Or reboot? Not saying that you should have to, but it might help, if it's not processing rules well on the fly.
If that doesn't work, I would suggest just deleting the FF rule entirely, reboot, run FF and make the same change you mention above, reboot, run FF and try to download.
LM