Hi labgramma, this is a temporary problem.

A critical Add-on-related certificate expired (which obviously should not happen). As a result, all verifications for that certificate are failing, preventing many extensions from being used, updated, or installed. So it's not your connection; it's everyone's connection.

The Add-ons team is working on a fix for this. Official updates are expected to be posted here:

https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/39047

Currently, the only workaround for the regular release of Firefox is to set your system clock back (for example, if your date is May 3, set to May 2; if your date is May 4, set to May 3). You might have to reinstall extensions after that, or maybe Firefox will detect the difference at your next shut down/restart. Changing the date may have other consequences, so I understand this option isn't ideal.

labgramma said

there is a message that it has disabled my Kaspersky antivirus

Your Kaspersky antivirus software itself was not touched at all as the message was referring to the Kaspersky extension that was installed in Fx.

Mozilla has rolled-out a fix for this. The fix will be automatically applied in the background within the next few hours, you don’t need to take active steps.

In order to be able to provide this fix on short notice, they are using the Studies system. You can check if you have studies enabled

1. Go to [=] > Options > Privacy & Security.
2. Make sure Allow Firefox to install and run studies is check marked.

I you had it disabled, you can disable studies again after your add-ons have been re-enabled.

They are working on a general fix that doesn't need to rely on this and will keep you updated.