I tried using PGP in Thunderbird on Linux. It was a pain. E-mails suddenly didn't format properly, and I got weird error messages at unexpected moments.
We need better user-agents for dealing with encrypted email.
You may be failing to start gpg-agent properly. Are you exporting GPG_AGENT_INFO? What does it say?
I noticed this was somewhat of a sore spot for people when Enigmail started forcing gpg2 instead of letting the user choose. I don't disagree with forcing gpg2, but there were definitely some configuration changes that needed to be made on my system to cope with / reflect that choice.
We need better user-agents for dealing with encrypted email.