How to download videos using a browser add-on or extension
Many browsers support the ability for add-ons or extensions to be installed to download streaming videos.
Copyright and Fair Use
Downloading streaming videos is not illegal, as long as you use the video in a manner that conforms to the video's license or terms of use. For example, many YouTube videos use the standard YouTube license, and some are licensed under Creative Commons. Check the license type by inspecting the information box underneath the video, as in the image below.
For more information about viewing and using YouTube videos in accordance with copyrights, make sure to check Google's Frequently Asked Questions about Copyright and Fair Use on YouTube.
Download videos using a Firefox add-on
To download videos (including YouTube) using Firefox, we recommend the add-on called YouTube Download Plus. To install the add-on, follow the install process below.
In Firefox, navigate to the YouTube Download Plus add-on page, and click Add to Firefox.
When prompted by the browser to confirm installation, click Add.
The next time you go to a YouTube video page, you should notice a Download button below the video.
Click the red download arrow 
at the top-right corner of the Firefox window to select and download the video as an MP4 file.
You are presented with a drop-down menu, where you select the option to download the video as an MP4 file. After selecting the MP4 option, several pages open and close, which are part of the MP4 conversion process.
On the "conversion finished" page, click the green Download button to save the MP4 file to your computer. After the download is complete, the file is available in your downloads folder.
Download videos using Chrome extensions
Google does not allow any extension in Google Chrome to download YouTube videos. However, the example below does allow you to download videos from most other video sites.
For Google Chrome, we recommend installing the Video Downloader Professional extension. To download this extension, follow the install process below.
Visit the Video Downloader Professional extension on the Chrome web store.
Click the Add to Chrome 
button once open.
Click Add extension.
After installing the extension, if a video is available to download, a green down arrow 
appears in the top-right corner of the window.