There's code hiding page action buttons, because they're duplicated in the menu, in order to keep the address bar is usable in narrow windows: https://searchfox.org/mozilla-central/rev/3ff133d19f87da2ba01ade55d6e7fdf0fc28b08c/browser/base/content/browser.css#764-774
Bug 1691454 Comment 2 Edit History
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There's code hiding page action buttons, because they're duplicated in the menu, in order to keep the address bar usable in narrow windows: https://searchfox.org/mozilla-central/rev/3ff133d19f87da2ba01ade55d6e7fdf0fc28b08c/browser/base/content/browser.css#764-774