Hi yursokolow, Kaspersky software includes a feature to intercept and filter all of your web browsing. As a "man in the middle" the software needs to generate fake site certificates but Firefox is not fooled. If you want to keep this feature enabled, you can set Firefox up to trust Kaspersky's fake certificates. There are two methods described in this article: https://support.kaspersky.com/14620 If you do not want the software filtering your browsing, you could try this: https://support.mozilla.org/ru/kb/kak-ustranit-oshibku-s-kodom-sec_error_unknown_iss#w_kaspersky