It appears that disabling the "Use Paragraph format instead of Body Text by
default" option is not persistent when quitting & relaunching Thunderbird.

Steps to reproduce:

1. Install Thunderbird 52.8.0 64 bit (on openSUSE Leap 15 in my case) from the
default repo or Mozilla repository
(https://download.opensuse.org/repositories/mozilla/openSUSE_Leap_15.0/mozilla.repo)
- both exhibit the problem.
2. Add a GMail account using the default options.
3. Preferences -> Composition -> Untick "Use Paragraph format instead of Body
Text by default". Confirm.
4. Write a new message and see the text format is set to 'body'.
5. Quit Thunderbird.
6. Re-launch Thunderbird.
7. Write a new message and see the text format is now set to 'paragraph'.
8. Preferences -> Composition -> Note that the option "Use Paragraph format
instead of Body Text by default" is now ticked again.

I can duplicate this on Leap 15 KDE and Gnome. The problem does not appear on
Ubuntu 18.04. All tests were done with the default installation of Thunderbird
52.8.0.

On openSUSE it appears the setting is written into the preferences file OK, but
Thunderbird does not appear to act upon this setting when it is launched (i.e.
Thunderbird acts as if this setting is 'true' despite the settings file stating
'false'):

opensuse-thinkpad:~/.thunderbird/wg9cqsd0.default> cat prefs.js | grep
paragraphuser_pref("mail.compose.default_to_paragraph", false);

I did originally report this to Mozilla under the following bug but after
comparing with Ubuntu is seems to be a Leap specific issue rather one with the
Thunderbird code.

https://bugzilla.mozilla.org/show_bug.cgi?id=1465830

Any help would be much appreciated! Thanks.