By one bullet you mean one bullet on screen, I presume.
A flamethrower in space is ... not what you think it is.
Your spaceship already has a flamethrower. It blasts out back and gives propulsion. Any other type of actual flames would not really work. Oh sure, one could imagine any type of chemical exhaust that somehow would self-ignite in vacuum without exploding the very exhaust valve it came from. And that would still be faster than the ship that spew it out front, instead of instantly beeing flewn through by the ship doing no damage at all, since it is just fire and your ship's hull does not care.
Easiest would be to just increase the existing propulsion stream for a short duration and make it do damage. You would fly to your target, take a sharp turn and blast it with your boosters. That is actually a maneuver you might read or see in science fiction settings. Sort of an improvised weapon.
If you want to install a front "flame thrower", the ship could stop for the duration of the blast, as it counters and mirrors the propulsion blast. It could even fly backward for a short time. Anything that is not concentrated energy and put out by your ship as some stream, is bound to move your ship, if it has high enough energy to be considered a weapon.
Something similar to a flame thrower in spirit would be a stream of plasma, contained by magnetic fields. Which would also do more damage than a mere flame. The effect might look similar to an electric arc traveling along two wires that are V shaped. The reasoning is, you might want to hand wave why your high energy "fire ball" would not travel further and has short range. And most things you would fire as an actual weapon would just do that in space. But if you need to contain the plasma in a field generated at your ship, outside of that field, it would just disperse quickly.
Explanations aside, instead of one "." flying over the screen, "shooting" out something like this, should do the trick:
o oo ooo oooo ooooo xoooo xxooo xxxoo xxxxo xxxxx _xxxx __xxx ___xx ____x ______