Well, generally speaking "firing" reduces the amount of unemployment insurance you get, while a layoff gets you the full amount. So an employer that shrouds a layoff in a firing is screwing the employee twice.

I generally think that the employer should have a very high bar on proving that it was performance based when you are laying off significant % of the workforce at one time. Every time I was involved in an employee firing, it happened out of phase with the typical performance review cycle, and it was typically one person at a time.