Cerebral palsy or physical disability more common in firstborn child?

No. Birth order is not a specific risk factor for cerebral palsy. It is however more likely to occur in children born prematurely.
Not birth order. No, the birth order has no bearing on physical disability or cerebral palsy. There are many other variables that come into play for that type of morbidity (e.g., birthing, trauma, infection, inherited disorder, etc.).