is it safe to drive a vehicle when the front differential is bad
What do you mean by bad? If it is howling or grinding you won't get far and you may be stranded in a bad place at a bad time.
If the front axle were to lock up you could end up in an accident.
FoR is indeed correct. Yet if it's just noisy from a shaft bearing you may be ok for quite awhile. I assume you've also differentiated correctly between front wheel bearing noise (fairly constant) and pinion-gear noise (on or trailing throttle correlated whine). Either of these can be allowed to steady-state on a use plateau fairly longterm. But if the noise increases steadily and rapidly I'd deal with it soon.
A mechanic friend of mine ended up in the ditch because a differential on his Subie locked up. I think it was the rear one though. The front would be worse.
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