Wired provided a public notice yesterday warning players to manually save their Halo 3: ODST campaign before hopping over to the Firefight mode. Apparently, the game's autosave doesn't really care that you don't want to leave your friends waiting on you.
Please do remember pause the game and save before accepting invites to other games or game modes. If your friends are impatient, you can jump into their Live Party and chat with them while you are saving your campaign progress. We're gung-ho for Firefight, too, but we have a city to save, dagnabbit.