If you have iTunes , the new version 11 allows you to convert their m4p files to mp3. Create a column "Kind" then sort by m4p and create mp3 versions. The old way was to burn iTunes about 20 at time to a CD and re - import as mp3 in iTunes.
Im afraid I don't have itunes. I've just downloaded and used FF and the original m4b file was over 5 hours and the FF version is mp3 but only 59 minutes long? More importantly, it just plays as a high frequesncy screetch sound?