Are there any other toolbars listed in View > Toolbars?

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Thanks, makes life interesting. Safe Mode, Default Theme with Bookmark mode has no blank line. Normal Mode, Default Theme has the extra space with a line in it. Normal Mode, Bookmark Bar, Default Theme has the extra blank space. I updated all of my extensions.

I have fixed the problem but not via the Add-on page. Under View->Toolbars, I used Customize and did a reset toolbars. then turned Bookmarks back on. Seems to work with Default & a basic color theme. No idea what added that line in there but ... it is now working. Thanks for your help

Back to your original theme. You might try playing around with a couple of things and see if there is a combination that works. A bit simple but that is always the best place to start. The first one puts the tabs above or below the bookmarks toolbar.

Various combination of -

Right click on toolbar space>check and uncheck Tabs on Top, and -

Options>Tabs>Check and uncheck Always show the Tab Bar.