The safest way is to extract it from the macOS installation app (Big Sur in your case).
You must use the original SSD to update such an old version of firmware. This is an old Apple requirement for your model. EBay used to sell them sometime ago, you can check if they still sell them.
If you have an original SSD, just by installing the latest version of Big Sur, your firmware will be updated.
Also, you can update the Firmware using a ROM writing device. You can find its description in the starting pages of this thread. In my opinion it is easier to buy a used original drive for the firmware update than such device.
In addition, the latest firmware version for your model is 478.0.0.0.0 (released with the latest versions of Big Sur - I don't recall which one first brought it).
You can use SilentKnight to find firmware information besides other information about your model.
https://eclecticlight.co/wpcontent/uploads/2024/09/silentknight211.zip)
Finally, your model is not compatible with Mojave. The latest compatible macOS is Big Sur. If you are running Mojave it is because you are using one of the patchers available to allow the use of newer macOS on Unsupported Macs such as OpenCore Legacy Patcher or using -no_compat.