OK, you do not say what system you have but I guess it is a 7029 / 9114 power 4 system from the firmware level.
You do not need to update the firmware to go to AIX 5.3 TL05 but you should update it anyway. You should update it to the latest, and only available level, for two good reasons.
1. Your level has bugs fixed in later levels.
2. If you hit an AIX bug and need to go to a higher TL in a hurry to fix the bug you may well need to update the firmware to accept the higher TL, and having to update the firmware and a TL in a hurry is no fun at all.
The power4 firmware is quite stable now so get it up to date while you can plan the firmware and TL update rather than trying to do it in the middle of a working day because you need to get a system working again in a hurry.
The TL and firmware requirements are listed in the AIX release notes:
pSeries and AIX Information Center
To download and update the firmware go to Fix Central:
IBM Support: Fix Central
Product Group: System p
Product: Firmware and HMC
Processor type: POWER 4 and earlier
Firmware type: System and device firmware
Continue
In option 1) Download System Firmware and associated Device Firmware for the selected Machine Type-Model: Select your system type (lsconf or prtconf will show you this if you do not know.
On the next page select the continue button.
Then you will see a list beginning with the system firmware, the Desc link will tell you how to install it.
You should also update the adapter card firmware and the disks, CD drive and tape drive firmware if updates are available for your system.
Each one has a Desc link telling you how to install it.
You will not need all of them, you should check what is installed in your system and then check which ones you need.
Tick the boxes next to the ones you want and then at the bottom of the page select continue and then follow the instructions to download them.