Hello! Our help will be more effective, if you copy and send us info about your browser and the Savefrom.Net extension:
http://en.savefrom.net/user.php?info=1
We'd also need some links you can't download (to test them). 

Do you download via the extension SaveFrom.net Helper or the website SaveFrom.net?
For the better understanding it'll be great to see a screenshot with the error.
We are waiting for your reply, and thank you for contacting us Savefrom.net Team!

Hello, in order to download MP3 from Youtube you need to install our program Ummy Video Downloader,
Also with the help of it you can download videos in high quality with sound: https://videodownloader.ummy.net

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

What kind of quality do you download?

Hello, after the playback is showed click with the right mouse button and save as.

Hello, after the playback is showed click with the right mouse button and save as.

You have two of our extensions installed.
Remove all that is associated with us, remove Tampermonkey.

Install the extension on these links.

Install the extension here in these links.
Google Chrome. First 1 then 2.
1. Tampermonkey black: https://chrome.google.com/webstore/detail/tampermonkey/dhdgffkkebhmkfjojejmpbldmpobfkfo?hl=en
2. Savefrom.Net: https://download.sf-helper.com/chrome/helper.user.js?ts=1455972944179

Clean the cache and cookies in your browser.

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

Hello, Yes, we are aware of this issue. And doing our best to fix it ASAP.
Thanks for using our resources.
Best, SaveFrom.net Team

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.

Hello, Reinstall the extension after an older version is uninstalled. The current version is 7.54:

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