Encryption should be automatic
PGP never took off, because it requires a public key server to verify identity and identity simply has nothing to do with encryption. Those third party servers are just honeypots of 'who is sending encrypted emails to whom and when' and places to attack if you want to substitute a key. They are the Symantec's of the world*.
Revoke key is not a good thing either. You let a third party say a key is cancelled because you trust that third party more than the actual email services you're talking too??? No.
Thunderbird should sent a public key with every email in the meta data. Mozilla should automatically collect public keys from the meta data and use them. It should have a setting "Automatically upgrade to encrypted", when it receives an encrypt key, it should use the key when emailing that address, and keep track of which keys were provided.
If you trust an email address enough to talk to them as if they were fred.bloggs@ then you trust their key to encrypt that message. You do not need a third party to verify fred.bloggs is fred.bloggs. If you didn't care enough for other purposes, then you don't care enough for the encryption.
Collect the keys, confirm the key remains constant, if ever the key changes, flash a big alert up "fred.bloggs's encrypt key has changed, is this still him?"
No third party keyholder, not 'trusted' third party. No Symantec's injected into private conversations. There is nothing special about wanting a private conversation in a communication system that is supposed to be private!
* And also notice that Google monitors its public keys, which is how they spotted Symantec issuing fake certificates, but you do not. You would not be aware if the 'trusted' keyholder starts issuing a different public key. PGP was never adopted because it is flawed.