thunderbird stuck with 'Status: Creating mail message...'

Solution 1:

Yes, it appears to be this bug: #687 Enigmail hanging on send - Thunderbird 52.1.1

Work-around

One suggested work-around (confirmed by multiple bug report commentators) is to kill all GPG processes,

killall gpg gpg2

and try again.

Beta-version with fix

You can also install the pre-release version 1.9.8-a3 of Enigmail that's supposed to contain a fix.