create custom_menupopup_check_icons.css

chrome/custom_menupopup_check_icons.css

@@ -0,0 +1,19 @@
+/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/custom_menupopup_check_icons.css made available under Mozilla Public License v. 2.0
+See the above repository for updates as well as full license text. */
+
+    menuitem:is([type="checkbox"],[type="radio"]) .menu-iconic-left{
+      -moz-appearance: none !important;
+      height: 14px;
+      margin-block: 4px;
+      margin-inline: 3px 8px !important;
+      background-repeat: no-repeat;
+      background-size: contain;
+      background-position: center;
+      -moz-context-properties: fill;
+      fill: currentColor;
+    }
+
+    menuitem[checked] .menu-iconic-left{
+      background-image: url("chrome://global/skin/icons/check.svg");
+      background-color: transparent
+    }

html_resources/tagmap.json

@@ -109,5 +109,6 @@
 "round_ui_items.css":["buttons","icon","menu","tabs","toolbars","bookmarks"],
 "curved_tabs.css":["tabs","tab"],
 "theme_aware_modal_prompts.css":["effect","dark-mode","colors","popup"],
-"centered_top_statuspanel.css":["status"]
+"centered_top_statuspanel.css":["status"],
+"custom_menupopup_check_icons.css":["icon","menu","minimal","popup"]
 }

tags.csv

@@ -109,3 +109,4 @@ round_ui_items.css,buttons,icon,menu,tabs,toolbars,bookmarks
 curved_tabs.css,tabs,tab
 theme_aware_modal_prompts.css,effect,dark-mode,colors,popup
 centered_top_statuspanel.css,status
+custom_menupopup_check_icons.css,icon,menu,minimal,popup