My certificate imported

Where did you import it to? If this is a personal certificate it should go into the 'Your Certificates' tab.

of course. Thunderbird doesn't seem to locate it even if it's been imported to "Your Certificates" and it has the same name and email(both in crt and account).

Did you import the cert and the private key?

christ1 said

Did you import the cert and the private key?

Do you mean combining public and private key into a single crt file?

Something like that.

Yes I did, in fact it automatically merges both public and private keys to some_file.pk12 file when using "openssl pkcs12" utility. I checked the content of pk12 file and it has both keys

I think the problem was being caused by the way how FQDN name was assigned to my certificate. Although I applied for <customdomain.com> certificate (without www or other subdomains), the certificate issuer released the certificate(by default) under <www.customdomain.com>(which is a subdomain). So in order for Thunderbird Certificate manager to locate a valid certificate the email should have been something similar to <name@www.customdomain.com>.

Are you saying it is working now?

I will try it once I get my certificate reissued.

p.s: Sorry for delay. I was in a travel :)