The Composition Toolbar is under View/Toolbars in the new message, or Write window, not the main window. Press Alt if the Menu Bar with View is hidden.

All the toolbars are ticked and showing in all screens EXCEPT the 'write' screen, Which means that I cannot send any emails because there is no 'Send' button - no buttons at all, but only on the composition screen. I have been using Thunderbrd for about 20 years with not one problem until now. But this one problem has brought the program to a halt for me. I have tried every which-way to get the mail bar back but nothing works. I have even uninstalled Thunderbird and re-installed it, but nothing has changed on the composing screen!

Try this: Help/Troubleshooting, click Open Folder or Show in Finder (Mac), close TB, delete or rename xulstore.json. Restart TB. Does the Send button show in the Write window?

Neither of the suggestions above work for me I'm afraid. My Thunderbird program is still at a halt for lack of the mail toolbar on the composition screen. I am at a loss as to know how/why it disappeared in the first place.