*** FIXED *** Thunderbird blocks image in signature

The Thunderbird Ver 52.0.1 and later blocks images added as absolute file path in signature. So every time when we open a new compose window, the Thunderbird asks to unblock each image, which is annoying. To fix this,

1. First open a new compose window and unblock the images as you do now, but do not enter anything in this compose window.
2. Then, from the File -> Save As -> select File...
3. Save this file to a folder.
4. Then, from the Tools -> Account Settings, choose the new file that we created in the above step.
5. Now, if you open a new compose window, your signature will be displayed intact and Thunderbird will not ask to unblock images.

Ref: https://support.mozilla.org/en-US/questions/1158319