I seem to have come across a way to substitute Entourage for Mail, but I don't think I've been able to dig up the link to see if it would work with Thunderbird. I did find
ZappTek :: iCal e-mail Notifications but that will not work for Thunderbird unless/until T-bird includes AppleScript support.
However if you're willing to use Lightning or Thunderbird 3, that might do the trick.