Rated 5 out of 5 stars

Really Awesome!
Heres a tutorial on how to get this baby runnin:
You need FFMpeg:
http://www.ffmpeg.org/download.html

Go there and chose the appropriate build, and download it. You will end up with a file called simply FFMpeg. Just put it somewhere you can find it. Doesn't need to be anywhere special, I put mine in a Folder called FFmpeg, and placed that folder in my music folder. Im assuming you already installed the plugin, so start Firefox, right click on the newly formed addon in the toolbar(i had to go to a youtube video for it to reveal itself), and click the options button. Under media conversion options, click specify location, and navigate to the folder, you DO NOT NEED to have the FFMpeg highlighted, so its OKAY if its grey, just navigate to the FOLDER and click OPEN. Boom. Now assuming you downloaded the proper FFMpeg codec from that downloads page. All should be Well. Great Plug-IN!

This review is for a previous version of the add-on (4.4.4.1-signed). 

You can also download FFmpeg from our site at http://www.completeyoutubesaver.com/ConverterMac.html

-Thanks for the great reveiw and tutorial!!