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Home page > Security certificate > Installation of certificate in Thunderbird
Download the certificate from https://netfree.link/netfree-ca.crt and save it anywhere on the computer.
Go to
Tools menu > Options... > Advanced > Cetificates Tab > View Cetificates > Authorities Tab > Import
In the newer versions:
Tools -> Account Settings -> Security -> View Certificates -> Import
In the dialog box, navigate to the location where you saved the certificate, select the certificate, and then click OK.
A window will appear, check all checkboxes and confirm.
You can add a more detailed guide here. Inspired by this: http://wiki.cacert.org/ThunderBird