It's too cold when ice stops being slippery. (This happened to one of the polar expeditions. They had trouble moving the sled loaded with their supplies.)Indoors, 78 is pushing it. Outdoors, I'm okay with anything up to about 90 as long as there's some shade.
How cold is too cold? (Indoors, I don't like it less than 75-76... outdoors, 65 is "cool", below 50 is downright cold! Yes, I'm a Floridian.)