So, I'm a regular human sitting in a completely dark room, without any sort of vision enhancement. But I have Thermosense Organs. Now, another regular human (this one without any sort of cyber, bio, or magic enhancement at all) walks into my 10-meter radius, and starts coming closer. I want to kill this guy.

Common sense, such that it is, seems to suggest that it should be easier for me to smack this guy, than it would be if I smacked him. Right? In other words, it should give me some sort of reduction to combat TNs for blindness, at least against anything with a heat signature. But in practice, it seems like it gives no bonus at all, or at least I can't find one in any of the books my GM owns (I'm still stuck in the SR2 ghetto, but he's running an SR3 game).

All I can see is that the "blind fire" penalty is normally +8, but with the thermosense organs you get a penalty of only... +8 (at least in the case of attacking invisible people). In other words, the thermosensors make it no easier to attack someone who is "easily detected" (their words). And that's just ranged combat, there's no mention about melee at all. IT's not vision-based, so using the "visibility modifiers" just doesn't seem to make sense.

I'm confused. Is there something I'm missing?

Well I suppose without the thermosense organs you wouldn't even know the guy was there, assuming you didn't hear him. At least this way you know he's coming and get the option of blind firing on him. Without them you'd have no clue and couldn't even shoot at him since you didn't know he was there.

I think they're meant for detecting people and other hot things, rather than being much use at actually targetting them. Still quite useful to have.

Correct me if I'm wrong, but don't certain additional penalties apply to "true" blind fire? Like, not being able to aim?

Thermosense organs give an abysmal sense of 'vision,' but at least you do know someone is there, and roughly where they are.

"Additional successes may provide information regarding the targets proximity, movement, and heat-output but will give few other details." The italics are mine.

Sounds to me like a good perception roll should give slightly lower penalties. Try and bleed a 1/2-point bonus per success out of your GM, maybe he'll go for it.

Just to nitpick, proximity and Movement do not tell you where the target is, you know he's there, and you know that he's moveing, but it doesnt say you know a direction... I probably wouldn't enforce that in my game though.

I'd interpret it like this. I'm guessing SR's Themosense Organs work just like the ones featured on pit vipers and other snakes who track by heat. Their Thermosense Organs basically allow them to ascertain the relative location and distance of their prey (because the Thermosense Organs are directional, they're mounted a little bit near where the 'nose' of the snake would be if memory serves. I'm assuming SR's Thermosenses function like this because 360 Thermosense would be something like...I dunno...informational overload?). The reason it works so well for snakes is because they hunt warm bodies against a cool desert background.

Now, let's go to your situation. You're hunting a warm body in an environment that probably has significant ambiant heat (Urban?), enough to mask the target a little but you follow the moving heat as opposed to the 'static' heat. I'd rule that you probably still have a + mod for shooting into the dark, but not at the +8. The mod would be environment dependant if you were just relying on your thermosenses.