Oh, I forgot because I'm tired: I figured out when the game leaks memory, and sent that information on to the developer. Basically, it happens when you switch screens after completing a challenge, including going from the challenge back to the challenge selection screen. I mostly went to the Achieves page after doing the challenges for each gun to find out how many rounds I had to fire, so if you copy it out in a Notepad file and just refer to that you'll prolong your crash-free playtime.
I also played with Task Manager open, so I could always see the memory charge Firefox was claiming. When it got too high, I'd close the game and reopen it, and all was well.
Also there's a second gun that flips: the PPK. It also has a smaller clip size than the USP, so you'll get there faster.
