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Thx!

yes you can,. only if the firmware is lower than 3.55 obviously

Upgrade to 3.55, definitely. Downgrade however (from 3.56 or higher), not so much. Not without some neat soldering skills and a progskeet or teensey++ board. Chances are that a new ps3 from the store will be on a 3.56+ firmware.
Although, there is talk of a 3.66 exploit in the works from Gitbrew.

I mean a completely new PS3. What firmware is a completely NEW PS3 on?

A brand new straight from your favorite retailer would probably on 3.70 or 3.66

if you go to gamestop. they have phat model ps3's that may be on lower firmware.