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

Please, send to me a link to a file that will not be downloaded.

the button is not clickable, button is calling, but nothing happens).

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

The extension is compatible with the browser of this version.
You can try a special version of the extension 7.15: https://download.sf-helper.com/mozilla/helper-sig_7.15.xpi

Delete all Savefrom.Net extensions that you have installed.

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

Have you cleared the cache and cookies in the browser?

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

Reinstall the extension after an older version is uninstalled. The current version is 7.40:

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

Clean the cache and cookies in your browser.

Ок.

Hello, in order to help you better I need the following information.
Please, let me know me the name and version of your browser and version of the extension,
all can be found using the following link: http://en.savefrom.net/user.php?info=1

Can you watch youtube videos in the browser before you are trying to download it?

This is not an error, the question mark shows instructions on how to use the extension, you can close it by clicking on the cross.