In general, to be "dismissed" means to be allowed to leave. For example: "The children will be dismissed from class at 3:00." If I were dismissed from work, I would assume that I still have a a job, but that I wasn't needed at that particular moment.
Although dismissed CAN mean fired, the term is more vague. If you want to fire someone, you should say "You're fired!," not "You're dismissed." (Just so there will be no misunderstanding )