A window air conditioner must be installed so that the bottom of its case sits flush on the windowsill. If there are pieces of molding or a wooden windowsill that prevents the air conditioner from sitting properly, the pitch of your unit may be improper. If a window air conditioner is pitched inward toward the house, water from the rain and condensation will flow back towards the house rather than away from the window.
Frigidaire window air conditioners typically have a user-serviceable condensation pan. This pan catches condensation as it drips from the coolant coils and drains it outside the air conditioner case. Remove the front panel of your air conditioner, and examine the condensation pan for any signs of damage. Instead of rain flowing into your home, the problem may be condensation from the pan, and the symptom only manifests itself when the humidity rises very high as it may during periods of heavy rainfall.
Most window air conditioners have at least one drain plug located at the back of the air conditioner case, which allows water to drain from the case when rainwater gets in through the side fins or other small ventilation openings. Ensure these plugs are not clogged or otherwise stopped up. Use a small piece of wire to clean out the drain openings, and allow water inside the case to drain out.
Frigidaire air conditioners typically ship with small accordion-style expandable side sleeves that act as a weatherproofing medium for the gaps in the window on either side of the unit. If rainwater is seeping in through these sleeves, beef them up with some weatherproofing tape and weather sealant.