"You're fired!"

Translation: 너 해고야!

[oee16]

너 해고야 = 너는 해고야

~(이)야 is the informal conjugated present form of 이다 = It is ~

해고 (noun) = dismissal (/walking paper/pink slip etc.)

너 해고야 = Lit. As for you, it's dismissal => You are fired.

[SvedishPlumber]

Why does this drop the 되?

[CF00l]

“되/해야” (must) is different from just 야, which is a verbal exclamation. Like instead of "You must be fired", it's just simply "you're fired!"

[JasonW98]

The phrase that you never want to hear

[AbeVallerian]

Why does it sound very similar to "너 내거야"?

[army23600]

Most casual one!!!

[freegilyeon]

당신은 해고야! ==> What's wrong?

[oee16]

I reckon:

당신은 해고야! (X) due to incompatible speech styles:

당신 as the formal polite pronoun "You" is used to refer to non specific listener(s);

while -(이)야 ending is used for 반말, "intimate/over familiar" style used amongst close friends and family.
For 반말 speech, better stick to "너" for "you".