Some possibilities: check View>message body as and see if it is marked for original HTML. Another possibility is to rightclick inbox, select properties and then select repair.

I thought I had posted a reply but it doesn't seem to have been saved. So what I said was - you are brilliant David and thank you so much. It was such a simple fix (your first suggestion did the trick). I am happier about continuing to use Thunderbird - although the 'clumping' of emails (threads) I find annoying. Is there a way to go back to how it used to be?

Well, with flattery like that, I am obligated to answer the next question... :)

Threading has been in Thunderbird since 2022 and ver 102. To many, it's a curse.

Here is my own documentation on threading, duplicating the above, but it may help: You need to highlight each folder and click View>sort by>unthreaded. If this applies to just a few folders, just repeat the process as necessary. If you have many folders or plan to add many new folders, you may wish a different approach, below:

More info: 
To change default for new folders, In config editor, you can change default that applies to NEW folders you set up:
mailnews.default_view_flags      0 =unthreaded  

Config editor is available at Tools>settings>general and scroll to bottom right. 
  ------------
	
These steps may also help: 
To set many *existing* folders, follow these steps:
This must be done on an account by account basis.
# select table view  - the icon just to right of 'quick filter' button above message list pane
#  select a message folder and apply all desired attributes - View>sort by from menu bar (if not shown, click alt key)
#  click the icon at right side of column display in message list pane
#  select the entry near bottom 'Apply current view to...'
#  highlight 'folder and children'
# move mouse left to see account name and move left again to display all account folders. Now CLICK. a small menu will appear to confirm setting all folders within that account.

I did a simple (okay, crude) video at https://davidkirk.org/SettingDefaultFolderView.mp4

This is going to take me a bit longer to read and 'digest'. I will have a go when I have more time and let you know how I got on. Thank you again for the help offered! It is 17.20 here! Actually - I am not sure what you mean by a folder - such is my ignorance. The emails clump when something decides the emails are following a theme or are a string of replies. So I want to apply it so that the emails come in separately and stay in date order. I'll leave it there for now.

A 'folder' example would be inbox, sent, draft, trash, etc. For a few folders, just highlight each, click view>sort by> and tick unthreaded. Do that for each folder. My longer post would be useful if you have many folders or plan to add many.

And it works!! I shall pass that one on to a friend who I know is similarly irritated by the threads! Thank you so much.

You are most welcome :)