First Loader is the Boot Code, also called Boot ROM, of the PS Vita. It is a short MeP executable run by the Cmep processor at boot. After initializing the most secure Cmep keyrings, it launches Second Loader.
See also the detailed boot process.
For all First Loader dumped at physical address 0x5C000, anything relevant is before the bss segment.
For a DEM-3000H First Loader, bss segment starts at 0x2A80.