With Firefox, what's the best option for extracting and downloading just the audio from a YouTube video (preferably in a mp3 format)
You may want to look at MediaHuman YouTube to MP3 Converter. It's a Windows program rather than being a FireFox addon, but it should do what you need. I use their YouTube Downloader, which can download YouTube videos, as well as being to able to download just the audio, and it works very well. I got the full version of YouTube Downloader as a giveaway. However YouTube to MP3 Converter is free.
I use 4K Video Downloader (another Windows Program). It allows for video downloading (obviously) but also allows one to extract the audio into either MP3, M4A or OGG containers.
If you don't want to install software or addons, you could use: http://offliberty.com/ Just paste the youtube URL and right click to save. If you later decide you want the video file, a button shows up that you can select, it will reprocess your request and provide a link for the audio .MP3 and the video .MP4
