transform.rotation is a Quaternion, not a collection of angles. Accessing individual components of a quaternion is almost never what you want to do.

Instead, you can do something like....

 Vector3 firingDirection = transform.right;
 firingDirection.z = 0;
 firingDirection = firingDirection.normalized;

 Vector3 velocity = speed * firingDirection;

transform.rotation is a Quaternion, not a collection of angles. Accessing individual components of a quaternion is almost never what you want to do.

Instead, you can do something like....

 // Implicit conversion to Vector2 strips out the z component.
 Vector2 firingDirection = transform.right;
 firingDirection = firingDirection.normalized;

 Vector2 velocity = speed * firingDirection;