Not a display of fire bombing at an airshow, but I’m sure there was a display by an Air Atlantique DC-3, spraying water
at an airshow. I don’t remember where or when though.

Displays of top-dressing or other forms of application of agricultural material are fairly common at NZ airshows. Normally specialist aircraft are used (like the locally produced FU-24), rather than anything representing the military types used for evaluation post-war.