- click settings>general and scroll down 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 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;             
}

unified-toolbar {
  order: 2;             
}

#tabs-toolbar {
  order: 3;              
}

Thank you David, that fix worked perfectly!

You're welcome.

This worked for me also. Many thanks!

But ALSO I had to look at Density and Font Size" to 'round out' how the new Thunderbird looks. (accessed via "View" menu pick).

My TB was re/set to 'Compact' in Density - when I've used / have had it on Default. Font Size is just weird, still working out the settings for me. Changing fonts may affect line spacing in template emails I've discovered.