Firmware v5.10 is the last official release for hardware v1a. Code has drifed so far from the humble v1a beginnings that we are no longer making releases.

Firmware v5.10 requires bootloader v4.x. The v1a hardware shipped with v2. You will need to upgrade bootloaders with a PIC programmer, no auto-update package was released.

For more see:
http://dangerousprototypes.com/docs/Bus_Pirate#Firmware_upgrades