The one you are most likely thinking of is either Sonic's theme in Adventure 2 "It Doesn't Matter" or "Escape from the City" from the subsequent level, City Escape. However, you are correct that the music that plays when Sonic jumps off the G.U.N. helicopter is indeed fire.
"Live and Learn", the main theme of Sonic Adventure 2, plays during the final boss fight against the Finalhazard; however, part of the guitar line is referenced throughout the game's soundtrack and a good portion of the opening melody is used in the semi-final boss fight against the Biolizard.
