Try right-clicking the 3 bar menu or a toolbar and selecting Customise toolbar. You may find add tab bar there. It apparently looks different on the Customise page. If you can't find it there either, press the button labelled Restore Defaults bottom right of that page.

https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

Are you possibly using code in userChrome.css that is broken and that needs an update?

• https://www.userchrome.org/what-is-userchrome-css.html

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems