As of August 2012, the Audiofire 8 will only work with FFADO if firmware 4.8 or earlier is installed in the device. Please refer to ticket #360 for more details and for news about progress in fixing this.
28 Oct 2013: Takashi Sakamoto has pointed out that firmware versions 5.5 and later no longer support 192 kHz sample rate. This is a hardware limitation and is not related to FFADO. It is not currently known why this rate is no longer supported. To continue to use 192 kHz sample rate, stick with firmware version 4.8.
29 Oct 2013: As of r2451/r2452, audio streaming with FFADO may now be functional with AudioFire-8 firmware versions 5.5 and later. This has not been explicitly tested by FFADO. Reports of tests are welcomed. This is thanks to Takashi Sakamoto.