Create rounded_menupopups.css

chrome/rounded_menupopups.css

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+/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/rounded_menupopups.css made available under Mozilla Public License v. 2.0
+See the above repository for updates as well as full license text. */
+
+/* Makes panels, menupopups, urlbar and searchbar rounded */
+
+/* Modify the rounding size here */
+:root{ --uc-menupopup-border-radius: 20px }
+
+panel[type="autocomplete-richlistbox"],
+menupopup,
+.panel-arrowcontent{
+  -moz-appearance: none !important;
+  border-radius: var(--uc-menupopup-border-radius) !important;
+  overflow: -moz-hidden-unscrollable !important;
+}
+
+/* The rounding is also applied to searchbar and urlbar to look consistent */
+searchbar#searchbar,
+#urlbar-background{
+  border-radius: var(--uc-menupopup-border-radius) !important
+}
+
+/* Reposition the panel-arrow  */
+panel[type="arrow"]{ margin-inline-end: calc(-10px - var(--uc-menupopup-border-radius)) !important; }
+.panel-arrow{ margin-inline: var(--uc-menupopup-border-radius) !important; }