My favorite firmware is 3.80 m33, it's very stable. If you're looking for features, I'd go with 5.01 m33.
To run CTF themes (custom themes), you need to download the psp application "CXMB", but make sure it's for the right firmware.
Emulator's:
1. GBA: UO Gpsp Kai (full speed on most games)
2.SNES: SNES9x (full speed on most games)
3.NES: NesterJ (full speed on most games)
4.N64: Daedelus X64 (full speed on a couple games, the rest are slow with graphical glitches)
5.PSone: Comes with the custom firmare, download the bin/iso to the memory card. (100% full speed for all supported games)
But remember: Go nuts, and have fun