I'm guessing it's not a big deal, but it was kind of annoying having cold water dripping on my feet today while driving in the hot hot heat with the ac on full blast. No rain.
That's surprising, there's no coolant plumbing on the driver's side under the dashboard. Maybe from rain water trapped in the cowl vent & dripping through a rust hole?? Or perhaps condensation leaking from the HVAC as Ahoragi suggested above. Is the condensation drain hose located low on the passenger/engine-side firewall possibly plugged & causing condensation to back up into the HVAC system? That hose is supposed to drip condensation out when the A/C is running on a humid day.