Franklin, given a reprieve on eight by Jono Boult at fine leg in the 23rd over, threw his wicket away for a well-made 75 shortly after, but the damage had been done.
With the Firebirds requiring eight runs from the final six deliveries, a mid-pitch conference was called to determine who would bowl the final over.
Scott Kuggeleijn, who had come through in the clutch for the Knights a number of times this season, was finally selected.
His first delivery to a charging Luke Ronchi was a waste high full toss, which the Black Caps gloveman calmly deposited - with some help from the Bay Oval outfield - past a despairing Trent Boult at deep backward square leg.
The next ball was in virtually the same spot and nearly carried all the way towards a triumphant Firebirds side that had got home with four balls to spare.
Ronchi (41 not out off 31) was classy in the pursuit of Northern's 232-9, helping the Firebirds to claim the Ford Trophy for the first time in 12 seasons.
For all the late drama, it was the Knights' finish with the bat that may have cost them.
Knights captain Daniel Flynn said losing five late wickets for 22 runs somewhat wasted the contributions of Brad Wilson (68), BJ Watling (50), Daryl Mitchell (30) and Mitchell Santner (44).
"A couple of guys got in and got started, but probably didn't kick on as well as we would have liked ... that's the way it goes," Flynn said.
"We still thought we had enough to make a game out of it and we proved that."
Northern Knights 232-9 (Brad Wilson 68, BJ Watling 50, Brent Arnel 3-30) lost to Wellington Firebirds 233-6 after 49.2 overs (James Franklin 75, Michael Pollard 43, Luke Ronchi 41*) by four wickets.
