The Commonwealth title was the only one to elude the para-swimmer to date, but she put that right with a dominant performance in the S14 200m freestyle.
The six-time Paralympic champion finished 1.54 seconds ahead of England’s Jessica-Jane Applegate in second to add another title to an already glittering CV.
“This has given me the full collection now,” Firth told BBC Sport afterwards.
“I am so thankful to be here and I am so glad my family can be here to watch me because they couldn’t be there in Tokyo [at the Paralympic Games].
“They do so much for me and I love them so much and I hope I made them proud.”
It was a great night for Team NI in the pool as Daniel Wiffen picked up silver in the men’s 1500m freestyle.
The Armagh man agonisingly missed out on a medal in Friday’s 400m final by 0.13 seconds.
But he bounced back last night finishing second behind Australia’s Sam Short to secure a third swimming medal for Northern Ireland.
Kate O’Connor secured another silver for Team NI in the Women’s Heptathlon.
The 21-year-old, who had started the day in second after personal bests in the 100m hurdles and 200m, had slipped out of the medal places after a disappointing long jump.
But an excellent throw in her strongest event, the javelin, and a fourth place finish in the 800m saw her finish second behind Katarina Johnson-Thompson.