Is this a high efficiency (90% AFUE or higher) hot air furnace? (Got a make & model number?)
The fuel type is...?
Does the flue have cap to keep rainwater out? (Could it be rain leaking through? If yes, a proper liner & cap should fix it.)
It may be exhaust condensate, but it shouldn't smell like antifreeze. (Oil fired condensate will have a bit of smell, propane & natural gas will look & smell pretty much like water.)
High efficiency furnaces & boilers have/need dedicated condensate disposal setups, which may be wholly or partially plugged.
If it's NOT a high efficiency system and you have exhaust condensation occuring something's wrong and it needs to be corrected before damage is done. Oil exhuast condensate is particularly corrosive and more critical to fix quickly, but don't let NG/propane condensation go for months either or you'll be replacing flue liners &/or heat exchangers or more.
If this is a boiler (and not a hot-air furnace), there are other ways water could end up on the floor.