I'm assuming that the fire particles, when spawned, are given a lifetime and a constant velocity based on the player's current direction, and then 0-20 degrees are randomly added to create a cone of particles. This has the effect of making the flames off center; the algorithm should work on a range of -10 to +10 rather than 0 to +20 which would then center the cone on the mouse cursor.
[0.12.20] Flamethrower needs negative angle offset
Re: [0.12.20] Flamethrower needs negative angle offset
Okay, fixed in whatever version will be the next – either 0.12.21 or 0.13.