Paris tells us she was panicked when she realized the ring was missing -- and says she accidentally left the massive diamond-encrusted piece in the vehicle that took her across the tarmac to her private jet.
How do you misplace a $350k piece of jewelry? P says she was exhausted because she'd performed in Ibiza the night before, then hopped a flight to Poland and worked all day ... before jetting back to Ibiza.
Bottom line ... she's extremely grateful to get it back, and says she's in the process of getting the firefighter's contact info so she can give him a reward, and send some gifts for his family.
She wasn't specific about how big the reward will be but, y'know Paris. She does everything big.