Nine years after Catriona Matthew became a major victor at the Lancashire venue, Georgia Hall followed in her spikemarks by also claiming the Ricoh Women's British Open title.
And a two-player race for the year's fourth major was definitely established when both picked up a shot at No. 4 and Pornanong followed Hall in birdying No. 6.
Hall - who was named in honour of the American state that hosts the Masters - also revealed she had spoken to former Open victor Tom Lehman, who won at Lytham St Anne's in 1996.
Twenty-two years later, she has her hands on one of the big five trophies in women's golf - remarkably her first LPGA title of any sort.
Hall kept it that way thanks to a superb par save on the ninth after finding a greenside bunker with her tee shot.
Hall, who receives a cheque of US$490,000, became the first English major victor since Karen Stupples won this event in 2004, and the fourth over all along with Laura Davies and Alison Nicholas.
The 22-year-old from Bournemouth is just the third home victor of the event since it became a major in 2001, following in the footsteps of Karen Stupples (2004) and Catriona Matthew (2009).
Thailand's Pornanong Phatlum recorded her first bogey of the championship on Saturday and ended the third round as she started it with a one-stroke lead at Royal Lytham & St. Annes.
The key stretch came when Hall made three birdies in four holes beginning at the 13th and took a one-stroke lead when she rolled in a 20-foot birdie putt at the 16th.
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A three-putt on the 17th green gave Hall a healthy cushion heading to the final hole, which could have been even greater had the Englishwoman's birdie putt not pulled up agonisingly short.
Hall, therefore, enjoyed a three-shot buffer playing the last and was able to three-putt for bogey and still win by two, Phatlum saving par from the bunker to safely hold on to outright 2nd place. The two-time major victor birdied three holes in a row from the fifth then added four more in five holes from the 11th to get to 13-under, three behind Hall and Phatlum.
Phatlum had led Hall by a single shot overnight and the pair jostled throughout an absorbing final day's play.
"I can't believe it is true", Hall said during a television interview just moments after the victory.
It did not take long for Hall to get her nose in front for the first time all week, a birdie from 20ft on the 16th taking her six under for the day. "I was so happy".
McIlroy finished the back nine with consecutive bogeys and never recovered.
Tiger Woods, an eight-time victor at Firestone, started 11 shots behind and figured he would go out with a bang by playing aggressively. Ian Poulter started three shots behind and shot 74.
"Things could have certainly gone better", Woods said.
"Winning this tournament has always been my goal for the year", Ryu said. "I'm just so grateful".