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You can drag the new tab (+) button to the Customize palette window to remove this button from the tab bar just like you do with other toolbar buttons.

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

Note that you can still double-click empty space on the tab bar to open a new tab, but you obviously can no longer drag a link to the plus button to open the link in a new tab.