Extensions need [[Extensions in Private Browsing#w_enabling-or-disabling-extensions-in-private-windows|permission to run in private windows]] when the browser is in [[Private Browsing - Use Firefox without saving history#w_can-i-set-firefox-to-always-use-private-browsing|private browsing mode]]. Go into [[Customize Firefox controls, buttons and toolbars|Customize mode]] and look for it there or click the "Restore Defaults" button in the bottom-right corner.

"Restore defaults" didn't help. toolbar and "customize toolbar" look as attached.

Click the puzzle piece Extensions icon and right-click > Pin to Toolbar.

See also: *https://support.mozilla.org/en-US/kb/extensions-button