In my experience, the monorail is closed during the fireworks and will not resume operations for at least 20 minutes after the fireworks end. If you watch the fireworks from the Tomorrowland side of Main Street, and had toward the monorail as soon as the show is over, you most likely be towards the front of the line. But the monorail won't be running yet, and they will not let you into the actual monorail queue. Because of that, the line backs up quickly and snakes its way through Tomorrowland, and looks like it's much longer than it truly is.
The last time we did this (and i was pushing a wheelchair, so bit even walking briskly), we waited about 10 minutes for them to resume operations, and then we got on the second monorail. Definitely beat the walk back to VDH.
Steve