The StudioLive Series III mixers have two firmware images stored in memory. There is the base Factory Firmware Image that is loaded on the mixer at the factory and is always present, and there is the Current Firmware Image, which is the current active firmware version that the mixer normally boots into and operates with, and that is updated/replaced when you perform a Firmware Update on the mixer.
The primary and essentially the sole purpose for the Factory Image is to provide a working firmware to boot into so that you can perform a Firmware Update.
NOTE: The Factory Image will almost always be a significantly older firmware version than the current firmware that is available for the mixer, and so for this reason, we do not recommend you revert to the Factory Firmware Image unless specifically directed to do so by Technical Support.
Reverting to the Factory Firmware Image does not erase and does not affect any of your User data like Scenes and Presets or other mixer settings that you have stored on the mixer. Those things are only erased by performing a Factory Reset.
NOTE: See here for StudioLive Series III Rack mixers
To force your mixer to boot the Factory Firmware Image:
1) Power off the mixer
2) Press and hold the Home button on the mixer
3) Power on the mixer as you continue to hold the Home button
4) Once you see the Presonus Logo with the boot progress bar, you can release the Home button
This forces the mixer to load the Factory Firmware Image and it erases the Current Firmware Image.
Because the only purpose of reverting to Factory Firmware is to have a functional firmware that allows you to perform a Firmware Update, once the mixer has booted and is functional, you should perform a Firmware Update to the latest firmware version, unless instructed otherwise by Tech Support.
NOTE: The Factory Firmware Image is likely to be a significantly older firmware version than the Current Firmware Image you were using and there may be newer features or fixes in the Current Version that are not present in the Factory Firmware version, so it is not recommended that you continue to use the Factory Firmware version.
It is also possible that the Factory Firmware is old enough that some mixer settings which were valid for the Current Firmware version are actually incompatible with the Factory Firmware version to the extent of causing problems in mixer function, or even preventing the mixer from booting successfully.
In the event that the mixer has been reverted to the Factory Firmware Image and will not boot successfully, you would be advised to perform a Factory Reset in order to clear out these settings and User data that may be preventing the Factory Firmware from booting successfully.