Sounds like you are viewing the full headers. Try this:

'Menu app icon' >'View' > 'Headers' > select 'Normal'

Thanks for responding. I can't even follow your advice though because the top line of the page, the blue bit with File, View, Tools, Help, etc has also disappeared and I can't find a way to put it up there! Sorry to be a duffer but none of this is in any way intuitive for me!

Have you tried pressing F10 to make the menu bar appear?

The 'Menu app icon' has three horizontal lines (see image below) and is on the same toolbar as 'Get Messages', 'Write' etc, so you do not need the top 'Menu Bar' toolbar enabled.

However, you can enable the top 'Menu bar' via a couple of ways. If F10 does not work try pressing the 'Alt' key to get a temporary 'Menu Bar'. Then set the 'Menu Bar' as permanent: 'View' > 'Toolbars' > select 'Menu Bar'

OR set it via the 'Menu app icon' Using version 68* Menu app icon > Options > Options > select 'Menu Bar'

Using version 78* Menu app icon > Customise > select 'Menu Bar'

Thanks for your help. I think recent update to T'bird 78.*** disrupted things and I didn't know about the F10 function! All now familiar and fine.