As Gant is pulling the Firefox out of the hangar, it is still very dark, as in pre-dawn. But as the Russian officers are shown shooting and watching the Firefox take off, it is suddenly much lighter outside as if it is after dawn and the sun can even be seen. But it would have taken 30-60 minutes for the light level to change that much.
The movie Firefox is a Soviet built radar-invisible aircraft. In reality, the American F-117 Nighthawk stealth fighter was made possible by the work of a Russian physicist, Dr. Pyotr Ufimtsev. (Soviet designers thought his theories worthless!).