Wellington's Dad's Army, with six players aged 34 years or older, secured the franchise's second domestic T20 title in three years at New Plymouth's Pukekura Park.
They beat Central Districts by 14 runs after losing the toss and posting 172 for seven.
The total appeared sub-par on a ground where a world record T20 aggregate of 497 runs were scored on December 21 between CD and Otago.
Yet the visitors' bowling and fielding excelled and they unleashed an arsenal of cunning and tenacity. Captain Mainwarings were everywhere to lead them out of strife. They appeared to work on the basis that if they bowled in a tight off-stump channel it would reduce the Stags' chances of freeing their arms to pepper the short legside boundaries.
It was a triumph for the tactical nous of captain Hamish Marshall and coach Bruce Edgar. Their charges held their nerve on a ground where sixes come to breed.
Mahela Jayawardene exited hooking in the first over (for a golden duck), George Worker went in the second (for two) and Jesse Ryder disappeared in the third (for a duck). That left the hosts' title hopes flatlining on six for three.
Oh, what they would have given for a Ross Taylor or Tom Bruce.
Batsmen from both sides were deluded by the thought they could swing for the mirage of the grass tiers. Most top players would fancy their chances of hitting a six with a stump on the postage stamp ground, let alone bats resembling two-by-fours. Ultimately, both sides struggled to combat regular wickets.
Of the older brigade, the bowling guile shown by 38-year-old Brent Arnel (two for 16) and 37-year-old Grant Elliott (two for 22) came to the fore as they tightened their grip on the title.
Hamish Bennett also delivered a masterclass with two for 34. That included a knuckle ball to remove Marty Kain who whisked 17 off seven. A timely fourth-over maiden from Anurag Verma built further belief.
A steady diet of wickets saw the Stags gain about as much momentum as a sherpa at altitude. Wickets in the ninth, 12th, 13th, 15th and 20th overs restricted any further scaling towards the summit.
In contrast, the Firebirds suffered setbacks in the second, sixth, 11th, 13th and 16th overs but survived through 37-year-old Michael Papps (29 from 35), 36-year-old Jeetan Patel (28 off 11) and Matt Taylor (48 off 25).
The odds were stacked against Wellington, given Central Districts had beaten them twice in the round robin. The Firebirds had also lost their first four games of the competition, albeit by nine runs or less on each occasion.
Wellington fell to eight for two after 1.4 overs. The visitors' boundary tent exhibited the joie de vivre of a morgue as batsmen waddled past teammates towards the distant dressing room.
Blair Tickner forced Marshall to spoon to cover and No.3 Tom Blundell offered a meek baseball swing to mid-off in preparation for his Black Caps debut. At 80 for five in the 13th over the visitors' chances looked grim.
Quality spells from pace bowlers Seth Rance (none for 33) and Tickner (four for 34), and stealthy spin from Kain (none for 23) and Worker (two for 20) saw them ratchet up the pressure, but Wellington's veterans secured the day.