3.56 can not be Jailbroken and there is currently no way to downgrade to a FW that can be, if your on 3.56 you should be able to go on PSN no problem
1) turn off with PS button menu, or through XMB
2) push and hold power till PS3 goes back off
3) push and hold power (again) you'll hear 1 beep and then 2 beeps then release power
You should be prompted to plug the controller (sixaxis, dualshock only) in with USB and push the PS button.
3.56 is the latest firmware, there is a DNS bypass program (fckPSN) that will tell PSN you have 3.56 and let you sign in (as long as you sign a new TOS, that says they'll ban you for any JB, CFW, homebrew, etc)