You got a sunroof?  There's these drain tubes that sometimes get
plugged causing water to overflow in your cars interior. Or possibly
your lower frt windshield wiper cover plAstic piece's seal is jus wore
out letting water seep in. Maybe your heater core went out.

I checked the passenger side floor when I bought the car last year,
and it was dry. I believe it stayed dry over the Winter, but after a few
weeks of frequent rain, I smelled mildew last week, and sure enough, the
front passenger floor was wet. Not soaked, but that was the source of
the mildew. I know about the issue with the firewall seam possibly not
having been sealed correctly, but if that's the case, how do I fix the
problem?