1.) This is not a complete app—it requires another "companion app" in order to function. This other app is not available through the Firefox extensions page and requests more access permissions. If I were a computer programmer/app developer, I could perhaps review for myself the source code for specific evidence of nefariousness, but I am not.

2.) I would have given 2 stars, but the .mp4s I was able to download were corrupted. Downloading and converting at the same time requires the companion app, and another, third app, as well. I will not be downloading two further apps from a website I don't know just because of the presence of this half-functional app on the Firefox extensions page.

Hence, 1 star, and uninstalling.