Well it's not ideal, but it assumes the computer it is stored on is securely protected, so Thunderbird would not be the weakest link here. Protecting this key

Well it's not ideal, but it assumes the computer it is stored on is securely protected, so Thunderbird would not be the weakest link here. Protecting this key would require to ask a password to the user. By default there are none, but users who use gpg are aware of security and would generally set a master password.