After a fresh install of Firefox, I had to find the Kaspersky certificate in Windows 10 and to export it. After, I imported it in Firefox (in "options" search for "certificate"). Before that, Firefox wouldn't open Youtube or Google, saying they were not trusted…
Why the hassle? AFAIK, Firefox can check certificates. It does so in Ubuntu, where there is no Kaspersky. So why can't Kaspersky just leave certificates alone and transmit them to Firefox? Is there any advantage?