It isn't the right time of year for it of course- but my favorite vantage point for the DC Fourth of July fireworks is on the steps of the Lincoln Memorial. It's a HUGE mob scene, but the view and the photographic opportunities are absolutely *stunning*.
The Reflecting Pool is in the foreground, and the lighted Washington Monument and Capitol Dome are in the background.
Mind-blowing, the first time I saw it.
Directly under the fireworks, looking up at them against the Washington Monument and the black night sky is another perspective I've enjoyed. It was interesting to compare the "Mall View" to that one.