Reinstall the extension: 

http://download.sf-helper.com/mozilla/helper_last.html

Clean up cache and cookie in browsers.

No, the Televzr program can download videos in this format: https://desktop.televzr.com

Hello, do you want to download high quality videos?

Hello, please give us more details.

For the better understanding it'll be great to see a screenshot with the error.

Hello! Due to the recent youtube changes, both our website and the add-on are enable to download in 720p.
We are trying to bypass it, but with no luck yet. For now, we can recommend switching to Televzr app, that can download in 720p and higher:
https://desktop.televzr.com/#

Hello, this site is not on the list of supported resources. The full list can be found here: http://en.savefrom.net

Hello, thank you for contacting support in Savefrom.Net
This bug is known to us, we will try to fix in the nearby future.

Hello, in order to help you better I need the following information.
Please, send me the information listed here: http://en.savefrom.net/user.php?info=1

Hello, thank you for contacting support in Savefrom.Net
This bug is known to us, we will try to fix in the nearby future.