1. Open Control Panel. For instructions, see Microsoft Help page. 
2. Open Internet Options. 
3. Go to the Content tab and click Certificates.

4. Go to the Trusted Root Certificate Authorities tab, select the Kaspersky root certificate and click Export.

5. Finish the export with the help of the Certificate Export Wizard.
   1. Click Next → Next.
   2. In the File name field, enter the full path to the folder to which you want to export the certificate. Enter any name for the file with the .cer extension. For example: С:\Users\Administrator\Documents\exported_certificate.cer

   3. Click Next.

   4. 

   4. Click Done.
6. Open Mozilla Firefox.
7. Click → Options.

8. Open Privacy and Security, scroll down and click View Certificates.

9. Go to the Authorities tab and click Import.

10. Select the certificate you got at step 5.
11. Select Trust this CA to identify websites and Trust this CA to identify email users. Click OK.

The Kaspersky root certificate will be imported to the Mozilla Firefox and Thunderbird storage.

1. In the Mozilla Firefox address bar, enter about:config.
2. Click Accept the Risk and Continue.

3. In the search field, enter security.enterprise_roots.enabled.
4. Click to toggle the value to true.

5. Make sure that the value of the string is true.  

Mozilla Firefox and Thunderbird will be reconfigured to use the system certificate storage.