by pushing the fire handle override buttons in order to release the mechanical locks.
This is also unnecessary as moving the CUTOFF switches to cutoff also unlocks the engine fire switches hence removing the need to press the override. (which are pretty awkward to use to say the least in my opinion!). The system is there to prevent inadvertent pulling of the handle on a live engine. The associated mechanical release is also unlocked in the event of a fire indication.
The FCOM lists the actions of closing the CUTOFF switches as
Closes the fuel valves
removes ignitor power
unlocks the engine fire switch
I believe the warning in the QRH is there to prevent crew 'regularly' using the override switches to cut the engine and to follow the correct procedure.