Haven't tried it, but it looks pretty forward. You may need to download and install Google Chrome, go to Chrome Web Store, and look for Stupeflix.com. Or, check-out their website using Firefox http://www.stupeflix.com.
Note: Although the basic, personal plan is free for unlimited videos, you have to pay for high quality or high definition downloads, and it contains stupeflix ad at the end. You can check the pricing below their website.