Look, yet again, heat signatures wouldn't really give you enough information. You'd know something was there, but you wouldn't know what until you were within visual range. And you wouldn't know if it was an enemy until it started shooting at you, or it failed to answer some sort of challenge. So you could track a ship going to mars clean from Pluto. What does that have to do with being able to use a heat signature in the middle of a dogfight? At high relative speeds with shooting going on, you need something with a shorter wavelength to get the data fast enough to be useful. I'm aware of the supposed ambient temperature of space. Even so, you've got a heat signature, and nothing else. It doesn't tell you anything about your target except location, and maybe velocity. Doesn't tell you who they are. "Captain, infrared scans detecting a heat signature in sector 2, on course x by y by z." That's all you've got. Are they friendly? Hostile? You don't know. Heat doesn't tell you jack.
As for visual tracking, are you telling me that on a combat spacecraft, you're going to try to put cameras all over it so that it can use image recognition to do that? I don't think so. Gun cameras are about all you're going to do, and they, generally speaking, just record things. They're not connected to the flight computer, which has much more important things to be doing than crunching numbers from images. Yes, people are making image recognition systems now, for LOW SPEED or STATIONARY objects. Processing video data is processor intensive. Even if you use thermal imaging in space to bypass the issue of visually being hard to see beyond combat range, you're still talking about something that is hard for a computer. Not only would it be tracking the enemy, but it has to factor in your changing speeds, your maneuvers, and how that changes the lead on the target, and thermal imaging, even in space, is not going to be particularly precise. Plus you still have to have a way of tagging specific craft as friendly or enemy, so you still need something besides the infrared, which really isn't going to give you a useful silhouette. Again, the wavelength is too long.
So maybe by the time of or near FreeSpace they've solved that. Maybe not. Maybe even with all their fancy tech. they can't fit enough of a computer on a small craft to handle that. Nothing is really said about it, but if these fighters have fusion power plants, they have to have a LOT of radiation shielding, plus the hull has to be fairly heavy as well, to survive stellar radiation plus radiation from heavy weapons. Doesn't leave a lot of room for the on-board computer. Plus, any combat pilot will tell you that if you make it so they have to pay attention to too many guages, dials, buttons etc., they'll die. So, given your usual military mentality, such systems are not going to be included as they are not effective, and they just create more cockpit clutter. Besides, despite the comments in game about Shivans being hard to detect, they're not. They're hard to track, yes, and at first impossible to get a lock on. Probably due to some level of EM interference from their shields. If it was active jamming we probably couldn't break it, given the tech superiority. You can fire heat seekers at them if they're in your reticle, and those will track, and even hit, but they don't deliver enough punch to deal with the shields.
BTW, Akalabeth, you need a refresher course on your physics. The data you get from tracking a thermal signature that started at earth and was heading to Mars while you're at Pluto would be hours upon hours delayed, but there's no time dilation. Each second that passed, you'd get the next second's data, at whatever delay you were at due to the range.