I wouldn't recommend using this. This will put a watermark on the majority of your downloaded videos unless you purchase a license. The biggest alarm is that this add-on requires you to download an external program (which it doesn't state anywhere in the description). The site claims that it's necessary and provides the source code via github, but no hash to verify it with the source code. Soon after downloading this, I started having some odd behavior with a service via svchost.exe causing high memory spikes, which I had never had since I've installed my OS years ago, and this was the only software I've installed recently in months.
I'm still checking whether the .exe of this addon was the cause, but I would be extremely careful and compile the companion app from the source code yourself if you're set on using this. There's another safer download tool called youtube-dl that is open source, well documented, and free. It's command line only but there's a gui front-end for it that also has instructions to compile directly from the source code.