Here's a short list of procedures to try if your Saffire firmware update fails repeatedly:
- Try disconnecting any and all other firewire devices from the firewire bus, leaving only the Saffire connected.
- Try updating both with and without the external power supply connected. 
- Pay attention to the LINE/INSTR LEDs on the front panel.  They should turn orange during the first stage of the update.  Once the first progress bar completes.  They should turn green, and then another progress bar will start.  Once the second progress bar is complete the LEDs should turn green and stay green.  If they fail to turn green after the first stage, disconnect and reconnect both the power and firewire cables from your Saffire.  This will allow the unit to reboot properly before completing the second update.