For the sake of this question, the firebending is set similar to the one of Avatar, except firebender cannot be a lightning bender. The setting is medieval, with villages, castles and such.
There are fire, earth, water, and air bender. Bender can manipulate only existing element, however firebender can always "summon" fire from his bodily heat and waterbender can extract water from the moisture.
Firebender can shoot jet of fire from any point from his body and maintain it. He can also fly with the jet. The strength and direction of the fire can be controlled when directly in contact with his body, while remote fire cannot be controlled (unlike other bender), so no reusing shot fireballs.
The protagonist is a firebender. He is about to join the intelligence team of the A kingdom. He is a natural fighter, street-proven as an orphaned child. The firebending ability is discovered just before he join the team, just as he reached 20.
The firebending ability is extremely rare, and so is other kind of bending in this world - usually a country only have 1-2 bender, so if utilized properly this can be a great asset to the intelligence team.
However, firebending techniques are a little bit... flashy, thus although unquestionably effective, not so well suited for assassination missions that requires stealth and anonymity.
Note : main concern is the undeveloped world means on pitch black night, if he used his firebending ability, it will notice people as it is easily mistaken for a fire breakout.
Update: It is important to hide the fact the assassination was done by a firebender. Benders are extremely rare. If the assassination method is only possible by benders, it can be traced back to kingdom who own a firebender (or other bender).
Due to politic (damn you politicians!), the intelligence team still want to keep him as part of the team, but unsure how to train him to utilize his firebending techniques while keeping his effectiveness while maintain the stealthiness when performing missions.
How can a firebender become an effective, but stealthy, assassin?
I'm looking for ways, including, but not limited to, techniques (limited to what firebender in Avatar can do) and equipment to achieve this.
Additional info: It is very unusual for benders to work together. Once a bender is discovered, different groups will try to recruit him/her and won't let go. In this case, this firebender is recruited to the intelligence team. Note that this does not invalidate existing answers, just additional info.