To do that, you can get “inside” the fireworks symbol to animate it by right-clicking on the listing for the fireworks symbol (you’ll notice it has a different icon than the graphic symbols) and clicking “Edit”. The rest of your scene will vanish and you’ll have a blank stage; the shapes/fills that make up your fireworks will be accessible. Above your stage you’ll see that your working area is now titled “Scene 1” (the main stage you’re working on) and then “Pink Fireworks” (in my case, anyway), signifying that you’re one level deeper and you’re now on the stage for your symbol alone. Your timeline will now have only one layer. To leave this area and get back to your regular scene, you can click the blue arrow to the left, or the text that says Scene 1.
Do not edit your symbol by clicking it in the scene. This will edit only that instance of the symbol, and the changes will not be reflected in the Library; when you drag a new copy of the symbol onto the scene, the changes that you made to the first instance will not be reflected in the new copies. If you can still see a “faded-out” copy of your scene behind what you’re editing, then you’re editing a copy of the symbol, and not the main symbol.
