Remember that they are in Ba sing say,?? Zuko would be quite foolish to firebend in the earth kingdom capital. So the fight would likely be fair. I still think Zuko will win though it will be a good battle. Also Jet may not plan on fighting him head-on or one-on-one he has a past of using traps to cause take out opponents.?? Longshot and smellerbee will also join in and who knows maybe the rest of the gang is already in Ba sing say??

You also have to put into account of how long and how many firebenders Jet has fought against. Since the start of the freedom fighters they've been doing everything they can to mess with the firenation troops. He will be a challenge I'll say that right now