Generally when the Magic Kingdom closes at 7:00 p.m. this indicates that there is some sort of special event going on in the park during the later part of the evening -- something like Mickey's Very Merry Christmas Party. In this case, there are fireworks that are part of the party and can be seen by guests outside the park. But in order to see the fireworks inside the park, you would need party tickets.
You can figure out the exact situation that will be happening during your trip, by checking the detailed information on the parks calendars. For example, the Magic Kingdom Calendar for November 2009 is found
HERE. Click on the information on the boxes inside the calendar grid for detailed information about fireworks times. For example,
THIS page for Friday, November 13, shows that there are no regular fireworks scheduled at the MK for that day.
THIS page for November 14 shows that Wishes is playing at 8:00 p.m.
Good luck with your planning.