This may be a long shot, but it’s what I do, so it might be an option: Set up a crypto gateway like CipherMail which will automatically decrypt inbound email and sign/encrypt outbound. The result is that your Thunderbird will never get to see an encrypted email, decryption is handled transparently before it hit’s your inbox. Obviously, if you don’t trust your email provider, this is not an option.

This isn’t simple and hence not for everyone, also comes with dependencies on your email provider, but it works flawless for me ever since I set it up. I run my own email server, hence adding in CipherMail wasn’t a big deal.