i dont know, here is what i thought of, to get the presence of fire, and its position, you would need 2 sensors, that mens that the fire is a the same hight of the robot and it only needs to turn left or right. put the sensos at some distance of the robot, so that the robot turns to the one that indicates more flames unti the indication is the same on both sensors and the firre is stright up ahead.
i dont know what you searched for phototransistors, they are still used in small circuits today, even is they are many years old.
there are also ir sensors that incorporate an amplifier too, but those are usually used for modulated ir beans for digital transimtion