If 78F is 25C, it is seriously tolerable. In France where we dont have AC, we spend the summer in front of fans and Im sure americans can use those too.
It all depends on humidity. I used to live in the US South where humidity would be >90% and 78F is unbearable. I live in Colorado now and the humidity here barely goes above 10-15% and the same temp is way more comfortable.
I live in Buenos Aires and 25°C is with air conditioner is perfectly tolerable. The official recommendation here is 24°(75°F), but at my home we keep the thermostat at 26°C (79°F) and sometimes more.
The air conditioner acts as a dehumidifier, so 26°C is nice, but you must change the water deposit of the AC quite frequently.
While I grew up in the US South, in a house without A/C. >90% and 78F sounds like a cool summer evening.
Hah, I'm right! In Tallahassee right now it's 82°F and 81% humidity, with a dew point of 76°F. At 5am tomorrow morning the temp will be down to 78°F. If you're out that early there will likely be fog in the low spots.