Same display in safe mode? Help/Restart with Add-ons Disabled. If this is a TB issue, not related to your OS theme or display settings, there is a way to adjust the spacing (line height) of the folders with a userChrome.css file. But that is a last resort.

Yes the problem disappeared when I restarted in safe mode and reset toolbars and control. Thank you

This is probably another of those weird Linux issues that traces back to an incorrectly encoded font in the GUI.

has there been an update to your windows environment recently? Gnome/KDE etc

Oh the problem has re-appeared. It still appears in safe mode and when I prevent the toolbars and controls. Yes I have upgraded to ubuntu 19.10 recently but I remember that I had the fault before I upgraded.

So, resetting toolbars and controls worked for a while, but the now the display is still wrong even in safe mode. This time, select Help/Troubleshooting Information, click Open Folder to open the profile folder, close TB, delete or rename xulstore.json (resets toolbars and controls), foldertree.json, mailviews.dat, session.json, and directorytree.json. Restart TB in normal mode and see if it restores the proper display.

http://kb.mozillazine.org/Toolbar_or_windows_layout_not_working

I am sorry, that didn't work either. I deleted everything you mentioned (e.g. folderTree.json etc) and restarted only to find the same issue. Finally I removed Thunderbird, purged it and removed everything I could find in my directories (sudo find -name etc) and then reinstalled it only to find the same problem!