Bethell, 20, and 23-year-old Mousley, combined to score 731 runs with seven half-centuries in 14 games to help Birmingham top the North Group table with 10 wins.
Bears lost by two wickets off the final ball in last year’s quarter-final at home to Essex and were skittled for 82 to lose by 104 runs at home to Hampshire in 2022.
They also suffered a last-eight exit in 2021 and have not reached the last four since 2017, when they lost in the Edgbaston final against Notts.
Bethell said: “We’re determined. Obviously, the last couple of results didn’t go our way. We had a really close game against Essex but we’re hoping to change that and play the same brand of cricket we have been playing throughout the competition.
“The boys are confident, everyone is in good spirits. The vibe from the Edgbaston crowd definitely pushes us.”
Mousley added: “When the Hollies is bouncing, there is no better place to play and we are all just looking forward to Friday, a good crowd, a good wicket and hopefully we can entertain the fans.
“It would mean a hell of a lot to get back to Finals Day, especially for the younger boys.
“We’re in a really good place, we have been really successful in the North Group, it’s about now converting that into a quarter-final – a home Finals Day would be unbelievable.”