firefox: explicitly specify serif as default font

conf/firefox-forbidden-policies.js

@@ -20,6 +20,9 @@ lockPref("cookiebanners.service.mode.privateBrowsing", 2);
 
 // I can set my preferred fonts in an order! :O
 // but obviously this is forbidden in policies.json!
+lockPref("font.default.el", "serif");
+lockPref("font.default.x-cyrillic", "serif");
+lockPref("font.default.x-western", "serif");
 // lockPref(
 // 	"font.name-list.monospace.x-cyrillic",
 // 	"Noto Shanns Mono, monospace",

etc/firefox/policies/policies.json

@@ -434,6 +434,24 @@
         "Type": "string",
         "Value": ""
       },
+      "font.default.el": {
+        "Comment": "Preference not allowed for stability reasons.",
+        "Status": "default",
+        "Type": "string",
+        "Value": "serif"
+      },
+      "font.default.x-cyrillic": {
+        "Comment": "Preference not allowed for stability reasons.",
+        "Status": "default",
+        "Type": "string",
+        "Value": "serif"
+      },
+      "font.default.x-western": {
+        "Comment": "Preference not allowed for stability reasons.",
+        "Status": "default",
+        "Type": "string",
+        "Value": "serif"
+      },
       "font.name-list.monospace.x-cyrillic": {
         "Comment": "Preference not allowed for stability reasons.",
         "Status": "clear",