Hi Nightowl,
Last year for France's Bastille day, Paris kept the Metro, Bus & Tram services running
an extra hour later than normal, with service stopping at 2h15 in the morning instead of 1h15.
Last year I attended the fireworks display which is launched from the Palais de Chaillot near
the Trocadero (opposite side of Seine from the Eiffel Tower) and the best vantage points are on the opposite side near the Eiffel Tower, in the park which is part of the Trocadero, or up on the street level behind the Trocadero. The Parvis des Libertés is completely blocked off, including the semi-circular tree area (where the name Trocadero is pasted on the Google Map linked above) so your vantage point from up on high of this area should about 20 meters or so away from the semi-circular tree area (or else the trees get in your way & block line of sight to the fireworks).
Public transportation services are absolute mayhem this evening after the fireworks are finished. I would recommend that you either stay put for the next hour or walk to your destination over the next hour as it will take you literally that long to simply enter a Metro station. I walked home from Trocadero to Montparnasse and it took about 45 minutes, which was faster than it would have been attempting the Metro. The majority of people did the same thing, although some decided to try waiting in line to enter a Metro station, at least those that were about 25 minutes walk away from the Trocadero.
Hope this helps!
Ben
ps. The fireworks were awesome even in comparison to The Celebration of Light in Vancouver, Canada where each summer we would have fireworks competitions with world class pyrotechnics teams putting together amazing displays.