For everybody having the failure "unable to rip mp3 unable to find file make sure you have pandora running in a FireFox browser." here is a possible solution:
The program seems to handle each file in the firefox temp directory and looks, if it is a part of the mp3 file. If not (in the temp dir are hundreds of temporary files with no context to pandora), it gets an error and continues with the next file. After (per default) 10 errors it stops trying to find the mp3 files with the error message "unable to rip mp3 unable to find file make sure you have pandora running in a FireFox browser." (even though firefox is used!).
I fixed the problem setting in the file "default.properties" the value of mp3.retry.count to 100. I think maybe you have to restart the pandora.jar server after some hours, but it worked properly without any failure.