Originally Posted by Gufsl1
But by flashing custom rom, I don't update firmware?
The ROM and the firmware are seperate.
The firmware usually contains hboot.
Custom ROMs do not include firmware.
The OTA includes firmware as well as a ROM patch and must be flashed together with ROM and Recovery both stock.
You can update the firmware manually.
If you want to flash a modified firmware (e.g., recovery & kernel removed), or a firmware for a different CID/MID/Part number you need S-Off.