Uncertainty kills morale very quickly. I think the most important thing is to be clear about why they were fired, so your well-performing employees know they are safe.<p>Of course, if you tell everyone you fired someone because of X, anyone else who does X will be worried. That may or may not be a good thing.
Address it early with people who need to know (direct team and people who work with this person frequently). Be honest without going into too much detail. If people press for details you can't or don't feel like sharing, tell them you can't share.<p>Don't make up a softer story to cushion it-- this can bite you big time. there have been companies that have been sued and lost because a manager used the "Tom left to spend more time with his family" white lie, because it masse jurors believe that Tom had been fired due to familial status (a no no).
Anything you say regarding firing an employee may end up being used against you in court, so it's likely not worth the time and effort trying to plan something like this unless there are mitigating circumstances that I can not possibly imagine.
"Today, Bob left us to pursue other opportunities. In this difficult time, I want to convey my appreciation for the dedication and hard work each of you brings to our team. Keep up the good work, and thanks for all the fish."
Usually in the US, everyone knows when someone gets fired. No need to announce. We just find out that the person is not there anymore since their desks are cleaned up secretly or they are walked out right away by security.