I don't know how well this addon downloads videos- I already have a dedicated Youtube video downloader- but what this addon did do is completely ignore every other video site I visited while also nagging me to download an extra piece of software (which mozilla naturally won't QC) and destabilize firefox to the point that it would frequently crash after roughly 30 minutes or so.

verdict: literally less than useless, because it adds nothing but also takes away functionality. uninstalled