Firmware updates never upgrade the Symbian OS or Series 60 versions. For the lifetime of the phone they remain the same.
At least today, firmware updates are not meant to be user-installable, but to be done by authorized Nokia service centers.
The Series 60 release functionality of a specific version that the phone is based on is just the starting point for device manufacturers, who can decide to add things to it as they wish (or for some things they can also remove functionality, if they so wish).