How can I make the Firefox menu bar dark?
The dark reader add-on doesn't change the Firefox menu color which is still white :
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The bar you are targeting for dark mode can be changed via a theme. Head over to Addon store for Firefox, pick a theme of your liking and install/enable it. Do note that many themes listed there won't work on Firefox for Android even though they would install just fine.
But even after this, the menu bar or whatsoever that is called when you press the three-dots line is still gonna remained unaffected.