To change the order of the menu bar and icon toolbar is to edit the CSS userChrome.css in the "C:\\\AppData\Roaming\Thunderbird\Profiles\.default\chrome\". If the "chrome" fodler does not exist (which should not), create it, and create two text files named "userChrome.css" and "userContent.css" (not sure if the latest is mandatory or not). Edit the "userChrome.css" file as below: