I don't think it's possible to move the tab strip via the customize toolbar option; so my guess is there's an extension that's modified your tab bar, or you've got some custom CSS running that modifies the browser's interface.

I would recommend you:

1. Try Launching Firefox in Troubleshoot Mode to test if the tab bar is in the correct place when in Troubleshoot Mode
2. Try Refreshing Firefox to reset whatever setting or extension is causing your tab bar to be moved

Here's instructions on how to do both of those things: Diagnose Firefox issues using Troubleshoot Mode Refresh Firefox - reset add-ons and settings

You can only move the Tab bar via code in userChrome.css. Troubleshoot Mode disables userChrome.css.

• https://support.mozilla.org/en-US/kb/contributors-guide-firefox-advanced-customization

More info about userChrome.css/userContent.css in case you are not familiar: *https://www.userchrome.org/what-is-userchrome-css.html

• https://www.userchrome.org/how-create-userchrome-css.html
• https://www.userchrome.org/firefox-changes-userchrome-css.html

In Firefox 69 and later you need to set this pref to true on the about:config page to enable userChrome.css and userContent.css in the chrome folder.

• about:config => toolkit.legacyUserProfileCustomizations.stylesheets = true
• https://support.mozilla.org/en-US/kb/about-config-editor-firefox