I don't know about the colours but here's how to get rid of Conversation View/threading.

Firstly press Alt-v and click "Sort by" on the drop-down menu that appears. Towards the bottom of that are "Threaded" and "Unthreaded." Click "Unthreaded."

That will sort out the folder you are currently displaying but only that one.

To make it work in every folder right-click somewhere on the line with the column headings and choose "Apply current view to folder and its children" at the bottom, then pick one of your accounts, then the top of the next list that comes up. See picture as hopefully that'll make it clearer.

You have to do it for every account you have.

Thankyou, That's all well hidden!! Thought they may have put it in settings!!! But got rid of threads, just got to find out how to go back to original colour scheme, the vivid Red is horrible. But thanks for the thread info. John.

Switch to the Light Theme under Add-ons/Themes. The System theme applies OS colours to buttons, menus etc.

Thanks, that worked. This should not have automatically changed after doing the update, original settings should have been adopted automatically.

Also. on your e-mail there to me there is a green button "MARK IT AS SOLVED" But when clicking it the following error message appears:

"This page isn’t working If the problem continues, contact the site owner. HTTP ERROR 405

Have to say that Mozilla is getting very complicated and messy these days. Its why I stopped using Firefox, almost every time you open it there is an Update of some sort, makes you wonder why so many updates, what's so wrong with it that needs all these. Not had the same issue with Chrome. The issue is becoming the same with Thunderbird unfortunately. Think you need to feed this back.