Ок :)

Install the extension from the Mozilla Firefox store: 

https://addons.mozilla.org/en-US/firefox/addon/savefromnet-helper/versions/

Hello, please, send us more details regarding the problem.

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
Could you please send a few links you can't download. We will check it out.
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.

And if you try another browser like Google Chrome?

Instructions for installing the extension: https://en.savefrom.net/faq.php#ext_inst_chrome

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
Could you please send a few links you can't download. We will check it out.
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.

Hello, it's not our fault that youtube decided to get rid of 3gp . There is nothing we can do about it.

Hello, there was an update with a bug fix.
Ummy Video Downloader OS Windows 1.10.4.​​​0
Ummy Video Downloader OS MAC 1.69
Please update or reinstall from our website: https://videodownloader.ummy.net

https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

At your discretion.

Reinstall the extension: https://en.savefrom.net/faq.php#ext_inst_chrome