Download VideoDownloader plugin for Firefox
Download your YouTube video

Use the following command to convert the source file to MP3

MP3 Conversion
==============
ffmpeg -i source.mp4 -vn -acodec libmp3lame -ac 2 -ab 160k -ar 48000 target.mp3

FLAC Conversion
===============
ffmpeg -i source.mp4 -f flac target.flac

MPG to MP4 Conversion
=====================
ffmpeg -i "in.mpg"  -strict -2 -f mp4 "out.mp4"

WMV to MPEG Conversion
======================
ffmpeg -i "in.wmv" -c:v mpeg2video "out.mpg"