No, the nightlies are not solely PC files, there are also versions for Mac and several Linux distributions. If you want to download the Mac-version be sure to choose the file with the .dmg extension.
I also use firefly on a Mac running OS X 10.4.11 and originally I also had the version installed that you downloaded using the link on the Roku side.
But with that version I sometimes had problems starting firefly (since I chose to manually start it).
Then I removed it using the included uninstaller and downloaded the latest nightly for Mac (svn-1696) which runs just fine.