That would be because of https://searchfox.org/firefox-main/rev/22adc0417a3dbf478c7af00d45daa311e2efdf8b/browser/themes/shared/urlbar-searchbar.css#67-74. We can probably pull that out for this case.
Bug 2038194 Comment 6 Edit History
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That would be because of https://searchfox.org/firefox-main/rev/22adc0417a3dbf478c7af00d45daa311e2efdf8b/browser/themes/shared/urlbar-searchbar.css#67-74. We can probably override that for this case.