I have always heard that ferrets shouldn'd be in temps above mid 80's.  I
believe it's that they don't sweat or pant, and can't release the heat.
I have noticed here on days when it's on the warmer side that they are
more lethargic.  Once I had them outside with me on a warm day and I
noticed they had their mouths open and were breathing kind of funny, like
panting but not - it was a labored breathing.  I got them into the cool
car quickly.  Freezing water bottles and putting them in a sock and then
near the ferrets can also help cool them when there is no a/c.
 
Dooks
Risa
[Posted in FML issue 4941]