How does Mozilla Thunderbird handles my key, and can it import directly from gpg keyring?

The answer is no. There is no way to have Mozilla Thunderbird utilize your existing gpg keyring for public keys.

Per this thread:

Public keys have to be imported into Thunderbird OpenPGP. There is no way around that. This is a design choice.

This is pretty mind-blowing. Can we go back to enigmail? Now we all have to maintain two distinct databases with the same content storing my contact's trusted keys and their level of trust :(

If we meet someone in-person and are able to verify their fingerprint and identity, we have to update it in more than one place. If we forget, then it's a nightmare trying to keep it all in-sync.