Fire stone does not boost turret damage. Each fire turret has a set amount of damage that is lowered by the enemies defense. Oddly enough, it says engineers use magic and engineering to attack... But that's just the description...
Also, fire turrets are pointless after you change classes. You will not have time to use fire turrets since you'll be destroying mobs as fast as you can lay a turret. If you plan to boss a lot (Bosses with craptons of HP), then a fire turret might be useful... But a waste of points in the end. A fire turret is also good if you wish to be a running meister in PVP.
The only turret that you should get is a push turret. It's good for last minute saves. You can use skills or a push turret in between comboes to stop the combo. Using a push turret would be wiser, though. This technique is especially helpful when a monster/player gets behind you and there's nothing left in front of you to attack.
"He who fights and runs away, lives to fight another day!"