Simplify the following text for a Grade 9 native English-speaking person.
Original: His most notable early performance was the world premiere of Igor Stravinsky 's ballet The Firebird , at the Ballets Russes , Paris , on 25 June 1910 .
Simplified: His most famous early performance was the world start of Igor Stravinsky 's ballet The Firebird , at the Ballets Russes , Paris , on 25 June 1910 .