well i would personally not recomend going back to 1.5 (if thats even possible for you seeing as how only phat psp's can support it) very few homebrews are released that are for that firmware version, and you would have to patch it quite a bit to be able to play newer psp games.
The firmware i personally use is: 3.9 m33-3 it seems to be the best for psx emulation, at least from what i found.
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