In Jainism, there is no “first birth” for a soul, because samsara is beginningless.

So the correct Jain understanding is:

• Every soul has been in many life-forms infinitely many times
• A soul has also been in Nigod countless times
• But we do not say there was one first birth from which everything started

So, in short:

No, not “everybody’s first birth was Nigod,” because Jainism does not accept a first birth at all. Rather, souls have wandered through Nigod and other life-forms since beginningless time.

If you want, I can also explain what Nigod is in Jain philosophy in simple words.