Basics:
https://bytesandbones.wordpress.com/2019/09/30/thunderbird-gpo-admx
Via Registry:
HKCU\Software\Policies\Mozilla\Thunderbird\Certificates\Install\1 = "mycert.der"
or
Software\Policies\Mozilla\Thunderbird\Certificates\Install\2 = "\\MYHOST\Users\username\mycert.pem"
or
Software\Policies\Mozilla\Thunderbird\Certificates\Install\3 = "C:\CERTS\mycert.pem"
as REG_EXPAND_SZ.
If you define only a file name without a path Thunderbird searches
%USERPROFILE%\AppData\Local\Thunderbird\Certificates
%USERPROFILE%\AppData\Roaming\Thunderbird\Certificates
for your files.
Via GPO:
Use the user branch. Define full path file location or file names only.
If you specify only a file name Thunderbird searches
%USERPROFILE%\AppData\Local\Thunderbird\Certificates
%USERPROFILE%\AppData\Roaming\Thunderbird\Certificates
ATTENTION:
With version 78.3.2 i run into problems importing binary (DER) certificates. I solved this case by using ASCII (PEM) certs.
I debugged this using STRG+SHIFT+J (Developer tools error console).