And as Jessica-Jane Applegate joined her on the podium for the second night running, Firth was overcome with emotion as she broke a championship record with 1:05.97.
"I only just made the qualifying time for this event," said the Northern Irish star.
"I knew I'd make the team but didn't think I'd be able to swim. It's been an amazing journey. I really think if you put your mind to it you can do anything.
"It's about surrounding yourself with positivity and people who encourage you, it just shows what you can do.
"It was tough out there and it's hard not to get things into your head but the crowd pushed me on to win it. It's so good racing in front of the British crowd."
Tully Kearney may have flashed a couple of glances at her mum and grandad but only had eyes for gold as she wrapped up a sensational double.
Backing up her 50m freestyle gold from the opening night, Kearney was untouchable in the 200m freestyle and revelled in the unique buzz of the London Aquatics Centre.
"I knew if I was going to win it then I had to start out quick because the Italian girls had good pace, so that was the aim really," she said.
"The noise of everyone really helps you keep going. I was looking at my mum and my grandad at the end and it's really good to see them here in the crowd. Seeing them spurred me on.
"It was absolutely amazing, the atmosphere in the pool is incredible. I could actually hear them during my last 25 metres and see them waving really kept me going when it got tough."
Kearney flew down the first 100m in 1:18.96 and touched at 2:46.92, a full 4.5 secs ahead of the field, breaking a World Championship record and getting Britain off the medal mark for day two.
The S5 star has won gold in two unfamiliar events. Her 200m freestyle personal best is a tenth of a second off the world record but she had never previously competed at a major championships in the discipline.
On another golden night for the host nation, Alice Tai broke the S8 100m backstroke world record in stunning style.
Tai won bronze in the 100m backstroke four years ago in Glasgow, but was incomparable over two lengths with the backing of a boisterous crowd in her adopted city.
Adding to 100m freestyle victory already secured at the Championships, the 20-year-old stopped the clock in 1:08.04 and broke her own world record by 0.05s. She was eight seconds ahead of the second place.
"It's crazy to get a world record. It's even more special because I've done it in front of a home crowd with my friends and family literally sat right in front of me watching," she said.
"It's lovely to get another gold. I'm not going to get too elated now because I've got six more events and it's about getting ready for those."
In the next lane was Megan Richter, who Tai shared the podium with at last summer's European Championships in Dublin, bringing down her personal best and winning bronze in 1:17.08.
Seventeen-year-old Maisie Summers-Newton scooped a maiden World Championship medal in the S6 100m freestyle, launching what is likely to be a long and illustrious career at the highest level.
The multiple European medallist came home in 1:13.87 to take bronze, just ahead of Paralympic champion Ellie Robinson.
After missing out on the podium by 0.05 on opening night, Stephen Clegg was 0.08 away from gold but still banked a scintillating silver in the S12 100m butterfly.
Jordan Catchpole played his part in a GB one-two-three on opening night and he helped the host nation dominate the podium again with S14 100m backstroke silver, Louis Lawlor winning bronze.