PS3 Firmware v4.21 is a minor update that improves the stability of the system software.
System Software Ver. 4.21
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Posts tagged "Firmware 4.21"
PS3 Firmware 4.21 Released
Sony have revealed that a new incremental update for the PS3 has been released. The update is optional, however, so you won’t be forced to update to it. Firmware v4.21 “improves software stability”, suggesting some games didn’t work well after the last update.