In the page action panel, the "Add Search Engine" action's icon is the page's favicon with a plus-sign (+) badge. But if you add the action to the urlbar, it's only the favicon, no badge. The reason is that the button in the panel is a toolbarbutton.badged-button, but the button in the urlbar is only an <image>. I don't think <image>s support badges, but tbh I'm not sure. It's probably possible to make it a toolbarbutton because as proof by example there's at least one toolbarbutton in the urlbar already, #urlbar-zoom-button. But it would require modifying BrowserPageActions because currently it always creates <image>s for buttons in the urlbar.
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