1. for (int i=0; i<9; i++) {
2.         if (bulletVisible==true) {
3.           bullet[i].display();
4.           bullet[i].move();
5.          }
6. }

in setup() ?

You should do this in the function spaceKey() . Because when the player moves it is not at the same position as it was in setup().

Also in the function spaceKey() don't use a for-loop.
Just make one more bullet visible. You can use a for-loop to

• find a bullet with bulletVisible == false and
• then set bulletVisible for this bullet to true.
• And then immediately leave the for-loop because you don't want to set two bullet's bulletVisible to true.

An arraylist would be better than an array btw.

Also if they leave the screen or hit something, set bulletVisible for this bullet to false (in move or so)

search for an available  isInactive Firework  object to launch() it again!