Are common fuses, like AT car fuses, temperature sensitive? Say a 12v 5A fast blow fuse, would it have any significant change in its reaction at 0ºF than it would at 100ºF? In either time to blow or current required to blow?
Yes, they are and there are usually curves to explain what the effect is. The current at which they open is probably affected a lot more than the reaction time to a short, which is I^2t related.
For example, here is the derating of the ATOF series of automotive fuses:
The time curves do not show any temperature effect, but you can estimate the effect by scaling the current to the equivalent at the new temperature. For example a 10A fuse at 120°C may behave similarly to an 8.5A fuse at room temperature. So a 30A overload might take ~0.1 seconds to open rather than ~0.15. That's just a rough guess though.
In automotive terms, I've never noticed any significant difference between fuses around freezing, and fuses at 100+ °F. If I short something with a fuse inline, the fuse blows, the electronics are fine.
Generally, a fuse blows up much faster than the other electronics on the circuit would, so even a 2-3 times difference is probably not important. For example, I've shorted unfused 20 gauge wire and it took human-scale times before it got hot enough to burn me. I was able to disconnect the wire from the switch without severe damage to the wire or switch even though the wire was hot enough to cause substantial pain in my fingers. There's no way a fuse would take that kind of time to blow in temperature ranges you're likely to see a car.
That said, I've never tested this on extremely sensitive components or tested with oscilloscopes and such. So your mileage may vary. I'd guess that the ECU in your car probably couldn't survive a short* that lasted human reaction times or more, but that the fuse is still more than adequate at any realistic temperature.
*As pointed out in the comments to this answer, a short on your ECU (or similar device) isn't inherently harmful to it. There are shorts that can damage the ECU, but if your ECU is drawing tons of current, it's usually because something inside already melted, so you probably needed a new one anyways. Normally speaking, the big thing the fuses protect is either against huge power spikes or shorts causing wires to melt and possibly catch fire.
Also, your ECU probably has very robust power regulators because car electronics are very messy, so the slight (~16%) difference between -40° (C and F are the same here) and 60 °C (140 °F) is unlikely to matter to the fuse. And you're unlikely to be much outside those ranges in your car.