Possibilities: If you have CCleaner on computer it might be cleaning up a file before the settings have been saved to the 'prefs.js' file. Stop CCLeaner from scanning Thunderbird folders.

Try: Locate profile folder: In Thunderbird

• Help > Troubleshooting Information
• click on 'Open folder' button.
• Exit Thunderbird - this is important.

• delete 'permissions.sqlite' - read comment 10 in bug report.
• Open 'prefs.js' file using eg: Notepad or similar text editor program. Note: you could create a copy of 'prefs.js' on desktop as a temp backup in case you make a mistake.
• look for preference: mail.ui-rdf.version
• Set the value to 14, so it looks like the line below - Note in Bug report they set Value to 14 and it successfully then upgraded to 17 .
• user_pref("mail.ui-rdf.version", 14);

• Save the 'prefs.js' file.
• Start Thunderbird.

Reset preference: Windows OS:

• Menu icon > Options > Composition > 'General' tab

MAC OS:

• Thunderbird > Preferences > Composition > 'General' tab

Linux OS :

• Edit > Preferences > Composition > 'General' tab

Then...

• uncheck "Use Paragraph format instead of Body Text by default".
• click on OK

Bug report:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1489976

Yes! That did it. Thank you!

BTW I didn't have the setting user_pref("mail.ui-rdf.version", 14); in the prefs.js file so I added it. Thanks again.

Good to hear it worked for you. Thanks for feedback.

Thank you, thank you, thank you!!!

This Paragraph/body text thing has literally been annoying me for years.

I went in and changed the option to 14, and when T-bird started it was switched to 17 and both replies and compose defaulted to body text.

I have cycled T-bird up and down several times and the unchecked status of the "Use paragraph format..." is finally being preserved across start/stop cycles.