Whenever you use Thunderbird on Linux you may encounter one annoying behaviour.
If you open a message which is got an attachment - you just need to open the message, not to save the attachment! - Thunderbird will place such attachment in our temporary directory, usually /tmp. You find the file there, and you think you used the Save as function to place it there, so you open it and you edit it.
Then you close Thunderbird, and poof! It clears its temporary files and deletes your modified attachment, possibily containing a day's work.
The workaround is simple: just use a launcher that changes Thunderbird TMP directory and use that to start Thunderbird!
#!/bin/bash -e mkdir -p /tmp/thunderbird TMP=/tmp/thunderbird /usr/bin/thunderbird "$@"
You may need to adjust the original binary path, then you can put such script in your path before your /usr/bin directory and you're done.