I should also note that I'm aware of the setting "Use Paragraph format" in "Tools->Options, Composition, General tab" and I have that checked. I like that and I want that, but the problem is that Thunderbird Changes the format without me telling it to do so.

Does this occur in emails you 'Reply' to or does it occur in new 'Write' messages?

Do you use a signature? If yes... Is it from a html file which is automatically added? Or is it entered in the signature box in Account Settings? Do you see a signature separator above signature which looks like --

Does this occur if you run Thunderbird in 'Safe Mode' ? https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

I don't have a signature.

The only two addons I have is "CompactHeader 2.1.5" which I don't need and I have now removed it. And "Extra Folder Columns" which is deactivated since it's incompatible with Thunderbird 52.6.0 and I have therefore removed it as well.

I did a test right now there I composed a new email and pressed <Enter> key two or three times to get to the text area without entering an email address or a subject, and then wrote some letters and pressed <Enter> key again and got this error. I then tried to repeat the process several times but was unsuccessful. So it happens sometimes but not always, and I have no idea on how to make it occur. It seems to happen in total random and that also makes it difficult to test safe mode. And these tests where done after I had restarted Thunderbird after removing the addons.

If I find a way to be able to repeat the error, I will try to do it in Safe Mode. But currently I don't now how to repeat it.

I looked into a similar complaint about this, where the user had selected body text. But if a numbered list or bullet point list is used, you need to hit enter twice to quit the list, and then it switches to paragraph mode. I found that in this situation, Thunderbird inserts a <p> tag, instead of the <br> tag that it normally uses in body text mode. This is a bug IMHO but I haven't raised it as a formal bug report yet; there is too much in the way of seismic shift going on right now in keeping Thunderbird working.

I hadn't noticed this bug myself. My habit is to just create the list as plain text and format it as a list retrospectively, by selecting it and clicking the list format button. But maybe if you can avoid hitting enter multiple times you might be able to stop the switch to paragraph mode. Maybe just inserting a single space character on each blank line might fool it into staying in body text mode.

i use an addon that shows viewing and editing the html source of a message. This often gets me out of scrapes where otherwise the cursor would be in the wrong place, with no easy way to get it back where I want it. This can happen if I insert a table or image and then find that it's not possible to add text below or above it.

I get similar problems in Word. I think these difficulties are intrinsic in the use of a rich text markup language. One of the early word processors, WordPerfect, got around this by offering a code view pane where you could directly see and edit the markup. I often wish that Word had a similar feature.