There is a known issue with the FireWire chipset used in brand new Apple "tower" Macs (such as the Mac Pro) when using units in the new Saffire Range. Although the unit will connect successfully and will look as though it is working, it won't pass any audio unless the Saffire is powered on before the computer is booted up. Powering up the Saffire before the computer is booted up avoids this issue.
Please note that if the Saffire is powered off and back on again for any reason while the computer is booted up (including the reinitialisation at the end of a firmware update), the computer will need to be restarted. Please ensure that any connected speakers are powered off before either rebooting your computer or power cycling your interface.
If you continue to experience issues, there may be a problem with your output routing in Saffire Mix Control. The 3-part Saffire Mix Control Tutorial explains how to ensure that Mix Control is configured correctly.