Use the icon, grid like on the top right. It is used to enter the tabs groups if you are on a tabbed window. The same icon on the tabbed groups take you back to the tabs.

See

• Mastering the Tab Group Editor
• Use Tab Groups to organize a lot of tabs

You can reset the browser.panorama.* prefs on the about:config page to remove the "Move to Group" entry in the right-click context menu of the Tab bar.

• browser.panorama.experienced_first_run
• browser.panorama.session_restore_enabled_once

• http://kb.mozillazine.org/about:config

Close and restart Firefox.