It seems that any addon related to youtube downloading gets zapped form the Firefox and Chrome app stores. :(

Here is one I have used on Firefox that disappeared:

Youtube Video Downloader - {a7847979-429e-4a40-8651-1a50fd8382ce}.xpi

If you want to audit it before installing it, change the extension to zip, and unzip it (xpi is just a zip file). It looks like it was created by someone french, but the code is pretty simple.