That's only the case if you're fired "for cause" which is a legally-meaningful term that's pretty rarely used unless you beat up the CEO on camera while snorting cocaine off a pile of embezzled $20s. If you're just let go normally (even if it's for being bad at your job), it's not usually "for cause" and you have 90 days to exercise your options (or longer if they're NSOs instead of ISOs).