Mozilla has stopped supporting PowerPC based Mac in Firefox 4. OS X Leopard (10.5) is the last operating system supporting PowerPC architecture. (Leopard also supports Intel architecture.)
Mozilla's Firefox support on Windows platform does not seem to be this harsh.
UserAgent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.7; en-US; rv:188.8.131.52) Gecko/20111103 AlexaToolbar/alxf-2.14 Firefox/3.6.24