Simmonds, a four-time Paralympic champion, took her S6 400m freestyle best to five minutes 18.69 seconds.
And Northern Ireland's Firth clocked 2:03.70 in the S14 200m freestyle, beating the 2:04.44 she set last year.
With one day of competition to go, 21 GB swimmers have achieved the qualification times for Rio.
Simmonds, who was disappointed after last year's World Championships, is attempting to win the 400m for the third Paralympics in a row.
"I didn't really want to go that fast at trials - I wanted to keep it for Rio," the 21-year-old told BBC Sport, after lowering her world record by 0.48 seconds.
"But I'm really happy to have pushed myself to that time and now I can work on the things I need to do in Rio, which is where it will really count.
"Everything is going really well. Last year at the worlds I didn't have the best of results and that has motivated me now.
"But I know Rio will be tough because Paralympic sport is moving forward so quickly."
Firth, 20, achieved her third qualification standard of the meeting after a battle with Paralympic champion Jessica-Jane Applegate, who also dipped under the required time.
Firth will finally make her GB debut at next week's European Championships in Madeira after switching nationality in late 2013.
"I wanted to give it my all out there and I did it," the Seaforde swimmer told BBC Sport.
"It was a really tough race and I was praying all the way to keep going.
"I could see Jess and she was going for the time and that encouraged me to touch the wall and keep ahead.
"The S14 category is getting bigger all the time and it is great that we can push each other on."
In the morning heats, Stephanie Millward (S9 400m freestyle) and Mikey Jones (S7 400m freestyle) added their names to the list of those to have reached qualification standards along with Tom Hamer (S14 200m freestyle), who broke his own British record in doing so.
Ellie Simmonds and Bethany Firth broke their own world records as they secured more Rio qualification standards at the British Para-swimming trials.