I deleted Firefox cookies and reinstall SF and the problem continues.
Then I tried to reinstall Firefox and still the problem persists and 
now that sign appears in red next to the download quality

Hi Aleksand!

This is the information about my browser

• Browser: Firefox 52.0 Windows
• User Agent: Mozilla/5.0 (Windows NT 5.1; rv:52.0) Gecko/20100101 Firefox/52.0
• Extension: ff-sf 7.57

This are one of the links that I can not download, but actually even though i tried can
not download any video

It does not give any error message, just the download do not start.

Thank you for your help. Regards

Julio