If you are familiar with userChrome.css, you might try this, as it works for me:

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

#toolbar-menubar {
  order: 1;             
}

unified-toolbar {
  order: 2;             
}

#tabs-toolbar {
  order: 3;              
}

If not, I'll add more instructions:

Hi David,

thanks for your answer. I couldn't find this userChrome.css file in my Thunderbird or it's profile. I have created one in "chrome" folder with the content you've provided, but nothing changed. I also moved it to profile folder, still no change. Can you explain me a bit more?

Thank you.

Okay. I probably should have done initially, but I was in a hurry: here goes - click settings>general and scrollodown to config editor - search for toolkit.legacyUserProfileCustomizations.stylesheets and change to true - then you need the chrome folder in profile, as you did, and a userChrome.css file, as you did. - 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

It works now! Thank you!

You are welcome.

did not work. could not find userChrome.css file

you need to create a folder named chrome in the profile and also create the userChrome.css file. My error for not being clear on that.

Great, worked for me too - why do they have to keep changing, at least give the option for long time users...