The new release of Thunderbird, codename Supernova, changes some key elements of the UI to be more "modern". For those who wish to keep the new release but modify some elements to semi-reflect the old UI, then here are some steps to accomplish this.

Restore Title bar

• Click the hamburger menu in the top right corner and select Settings.
• Click the General tab.
• Scroll to the bottom and click the Config Editor button.
• In the Advanced Preferences dialog, search for mail.tabs.drawInTitlebar and set it to false.
• Restart Thunderbird.

Move File menu to below Title bar

• Open your current Thunderbord profile folder. If not sure where that is then do the following:
  • From the file menu, click Help -> Troubleshooting Information.
  • Find the Profile Directory entry and click the Open Directory button.
• Find the chrome folder and open it. If the folder doesn't exist, then create one. Note the folder name is case sensitve.
• Open the userChrome.css file. If the file doesn't exist, then create one.
• Add the following:

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

#toolbar-menubar {
  order: 1;
}

unified-toolbar {
  order: 2;
}

#tabs-toolbar {
  order: 3;
}

• Restart Thunderbird.

Better spacing between elements

• Click the hamburger menu.
• Under the Density entry, select Default or to taste.

Better Toolbar display

• Click View -> Toolbars -> Customize from the file menu.
• Undert the Mail tab, set Button Style to Text Only or to taste.