- click settings>general and scrollodown to config editor - search for toolkit.legacyUserProfileCustomizations.stylesheets and change to true - then you need a folder named 'chrome' in profile, and a userChrome.css text file in the folder (with content shown below). - exit and restart TB and it should work, although I've sometimes found I have to restart TB twice to see changes in userChrome.css - Below is content for userChrome.css

/*  #######    */
@namespace html url("http://www.w3.org/1999/xhtml");

#toolbar-menubar {
  order: 1;             
}

Thanks for the suggestion.

I created the file, restarted Thunderbird a few times, and even restarted the laptop, but that didn't seem to work.

However...

I then did a bit of Googling, found a MASSIVE bunch of people complaining about the new look (which made me feel I wasn't alone in this, if not exactly better), and the solution: You just missed out the minus sign in the line order: -1;!

So, that's the menu order fixed. There are a bunch of other things that I don't particularly like about the layout, and there are threads on reddit suggesting further CSS changes, but I don't have time to try them out right now, so that's a job for another day!