I'm also a Mac user missing out on FlexbridgeRadio and it's a shame it's not available for us, but such is the way. The easiest thing to do is just fill your presets with the stations you listen to so next time Radio Roku goes down you can still access your preferred radio stations. Unless you're channel surfing a whole load, with 18 presets available you've probably got most of your based convered.
Shoutcast is an option but requires some custom install as it's not natively supported on Mac/Linux setups:
UNIX, LINUX, FreeBSD and MAC OSX:
1. Download the SHOUTcast DNAS application version appropriate to your system by proceeding to this link.
2. Gunzip the distribution and un-tar the archive.
3. Use a text editor to configure the sc_serv.conf file (Please see the SHOUTcast DNAS application section of the Documentation page for more information on what each field in this config file means ).
4. Note that the user you run the server as requires read access to the config file, and write access for the files you define for file storage and log storage.
If you're happy tinkering under the hood then great, if not, then you might want to avoid.
As for Firefly, this is a music server alternative to iTunes and so on (and a very fine one too) but does not deliver internet radio itself.
G5, OS X 10.5.8, 3 x M1000, iTunes 10.4, slimserver 6.2.1/alienbbc (now redundant), firefly svn 1586, uNSLUng 6.8