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

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

Thank you for using our extension SaveFrom.net Helper

Install the extension from our official website:

http://en.savefrom.net/user.php#helper_install

Reinstall the extension after an older version is uninstalled. The current version is 7.60:

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

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, 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

Go to Tampermonkey settings, enable Advance settings mode -> Downloads BETA -> Download Mode: browser api.
Restart TM – everything should be working.

Do you download via the extension SaveFrom.net Helper or the website SaveFrom.net?

Please, send me the information listed here: http://en.savefrom.net/user.php?info=1

Send a couple of links to the video that you download, for the test.

Hello, Unfortunately, we don't fully understand you... please. provide us with more details regarding the issue.