Possibility - Sounds like the session does not update necessary files after exiting Thunderbird. Do you use a product like CCleaner or have a similar 'clean up files' operation being performed by perhaps your anti-virus product ? If yes, then you need to make Thunderbird profile exempt from those programs or stop using them because they can clean up required files like cache, session and even passwords causing all kinds of silly issues.

No, I don't use CCleaner or similar software on this PC. I also turned the Anti-Virus (and tamper) off and it still resets. I checked config and when I restart Thunderbird the key has been reset to True. Even when I manually set it to false it resets on restart. Almost ready to revert to an older version of TB and not allow updates.

What OS are you using ?

Windows 10 Pro 20H2

Did this suddenly start when you updated from 78.5.0 to 78.6.0 ?

It was a much older version, 64?

So I changed the setting and exited Thunderbird. Made a copy of the prefs.js file and relaunched. The copy has the "mail.compose.default_to_paragraph" key, the prefs.js does not. So when Thunderbird starts it is either getting a new version from somewhere else or deleting that entry.

So after exiting Thunderbird, the 'prefs.js' file had this: user_pref("mail.compose.default_to_paragraph", false); but after restarting Thunderbird that 'prefs.js' file had deleted that entry.

Did you notice if Thunderbird had created a new 'prefs.js' file, in other words are there other 'prefs.js' files but with numbers in name eg: prefs-1.js, prefs-2.js etc ?

Workaround - as I think there is some weirdness abound... This next bit is an idea on how to force Thunderbird to set this option. Exit Thunderbird..... In the 'profile name ' folder where the 'prefs.js' file is located, do you see a 'user.js' file ? If no, then open 'Notepad' Enter the following - copy paste if required. user_pref("mail.compose.default_to_paragraph", false); Save file as : user.js in the 'profile name' folder, so it's in same place as the 'prefs.js' file. (If you use a Windows text editor, make sure you unhide extensions for known filetypes in Folder Options, so that the 'user.js' file isn't really called "user.js.txt". If you discover it is called 'user.js.txt' then right click on that file and select 'rename' and remove the .txt part. It will probably say are you sure blah blah it might not work etc.. say 'yes' as you do want to change it.

Then start Thunderbird.

No other prefs files other than the copy I made. Already tried the user.js workaround. File is there and doesn't get modified on startup, but also doesn't have any effect on the setting. And yes, it has the .js extension. Maybe time to try the 64 bit version? Or creating a new profile and see if the problem persists?

I would try a new download of win64.

Thunderbird program will be located here: C:\Program Files (x86)\Mozilla Thunderbird Delete the 'Mozilla Thunderbird' folder then empty the 'Recycle Bin' on computer.

Get a fresh download of the same version as available here on official webpage. Just choose the one for your preferred language. Choose the Windows64 bit version. https://www.thunderbird.net/en-US/thunderbird/all/

Please report back on results.

Install of the 64 bit version didn't change anything. Created a new profile and the setting stays. So there is something in the profile that is not allowing that setting (and perhaps others) to work correctly. Will do a bit more troubleshooting and post what I find.

Copied the prefs.js from my laptop to the PC and updated the calendar settings (I use Google Calendar) and the setting stayed. So there was something in the original prefs.js that was either corrupt or not allowing the setting to be saved.