User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.121 Safari/537.36

Steps to reproduce:

Went to Options then Composition.
De-selected the option 'Paragraph format instead of Body text by default'
Selected OK

Actual results:

As expected, this changed the default behaviour writing new messages (body text became the default)
HOWEVER, after Thunderbird was re-started, the changed default behaviour was lost

Expected results:

Thunderbird needs to remember newly configured behaviour for new emails in Options.

a) check that your prefs.js file is not read only.
b) is thunderbird totally shut down before it restarts?

The prefs.js file is not read only.
I first discovered I couldn't save this option yesterday and have had the computer on and off since.
One comment I will make is that I have two profiles. Not sure how!

I will just add that this does not seem to be a new problem. See https://support.mozilla.org/en-US/questions/1227317#answer-1172465

One of the later comments from November last year:

The newest and latest update to TB is flawed, and is in need of an immediate fix. It keeps defaulting to Variable Width and Paragraph when composing an email, and it CANNOT be changed.
Moziila MUST CORRECT THIS in a Patch.

To summarize, when composing a new email, it will automatically default to Paragraph instead of BODY TEXT, and it cannot be changed to keep it that way.
There is NO WAY in the newly-updated TB that just came out recently to do this, and is denotes a Coding error and flaw that the Code-Writers mistakenly put in there.
Please create a new patch and FIX THIS!!!!!

• try setting one or two other preferences. are they also lost on restart?
• try starting thunderbird in safe mode, set a preference, then restart

I changed the font and text colour preferences and these options were kept after restart.

Using safe mode did not change the paragraph problem.

In the problem mentioned in the above linked support forum, the final post suggested that the guy who was asking for a patch should try using the config editor to change the paragraph preference (mail.compose.default_to_paragraph). I tried that but it doesn't solve the problem. The changed preference was lost on restart.

Maybe there is a user.js with this pref that reverts the setting. user.js is only read on startup and has precedence of the prefs.js where the settings from pref window are saved.

We've seen this before. What happens is that your profile upgrade fails every time you start TB and the pref gets reset. Please follow the steps in bug 1489976 carefully.

Note that using uppercase in your comments will not accelerate anything.

Sorry about the upper case comments! They were included in someone else's report of the problem that I had copied.

Unless I am misunderstanding things, the bug being discussed here is indeed that reported 6 months or so ago but all the 'resolutions' have been marked as invalid so presumably the problem has still to be addressed.
Thanks to everyone who works on this mail program which is an excellent one.

Sorry for my last post. I was getting a bit confused with all the different different bug reports and references to being invalid.

I've now found the relevant thing needed to be edited - setting 'mail.ui-rdf.version' to 14 in prefs.js in the Profile.

The edit solved the problem! Thank you to everyone who works on the program.

I'm glad I could help you. Yes, you did the right thing.

The background is that TB keeps an "internal" version, so every time we make a change to certain internal structures, we increase that version and add an upgrade step. Somehow you came from a version below 14 and an upgrade step for versions below 14 upgrade went wrong. That seems to have happened in a few cases but we haven't worked out the cause. You may also want to follow the suggestion in bug 1489976 comment #10.