You can do so from their add-on listing page: https://addons.mozilla.org/en-US/firefox/addon/kaspersky-protection-2021/.

Hope that helps -- Ed

I have the same problem after reinstalling Firefox latest version after I had a disk crash. Before the disk crash I had the Kapsersky Password Extension on my version of Firefox that was up-to-date. I gave thumb-down comment on Edward Sullivan's reply because the link is for Kaspersky Internet Security extension which I could add without a problem. The issue is related to Kaspersky Password Manager, not to the internet security app.

It may not be available anymore per the below.

https://support.kaspersky.com/us/faq/2024-us-sales-statement#kaspersky-password-manager