Workaround for Thunderbird won't display new messages after upgrade to 38.1.0 or 38.0.1
July 2015: After upgrading to the latest version (38.1.0 or 38.0.1) Thunderbird won't display new messages and its error console shows a "weak ephemeral Diffie-Hellman key" error".
Here is a workaround:
On the right side of the Tool bar there is a button with 3 horizontal lines which 'Displays the Application Menu'.
Click on that and select Preferences -> Preferences.
Select the Advanced icon which should be on the right.
The first tab is General and the Config Editor button is on the bottom right.
Search for dhe, then change the settings for these 4 lines to false (double-click to toggle the value)
security.ssl3.dhe_rsa_aes_128_sha security.ssl3.dhe_rsa_aes_256_sha security.ssl3.dhe_dss_aes_128_sha security.ssl3.dhe_rsa_des_ede3_sha
Save and close Thunderbird.
The next time you open it it should be able to connect and show new messages.