Hello,
I think there is no firmware available for these newer (1TB/platter) models from Toshiba/Hitachi.
A bit of history, in order to have a clearer picture:
Before Toshiba took over the 3.5" production line from Hitachi, the Hitachi drives had a density lower than 1TB/platter and old 512-byte sectors (the 7K3000/7K4000 and 5K3000/5K4000 models). These models had a denomination HDSxxxxxxALxxxx and had firmwares usually ending in 580, 5C0, 800 or A10. We had access to these firmwares and could easily upgrade/downgrade.
Around the time Toshiba took over, the drives switched to 4K sectors and 1TB/platter density, with model names Hitachi HDSxxxxxxBLxxxx and firmware ending in AB0 ("Hitachi" is still shown in the OS, but we can see Toshiba branding on the physical drives). Since then, no more firmwares available for these drives. Later on Toshiba updated the model names reported to the OS to Toshiba DT01ACAxxx (7200 rpm) and DT01ABAxxx (5700 rpm) and newer firmware BB0.
So yeah, since Toshiba makes these drives, no more luck with firmware updates it seems...