why did this plug fall out of my 97 firebirds 5 speed?

This plug was there prior to the vibration issue I have now. There is now a hole on the back of my place where the shifter bolts down. I have never seen this plug just fall out,.

What size of plug originally went there?

do you mean the freeze type plug on the rear or sort of like a drain plug? If you mean the freeze type plug on the rear of the shifter pedestal, it's removed to be able to remove the shifter linkage.

this

Yesthe freeze plug on the back of the shifter pedastal.