diff --git a/src/display/font_loader.js b/src/display/font_loader.js
index 23f4a6a1c..7ca4ebbf5 100644
--- a/src/display/font_loader.js
+++ b/src/display/font_loader.js
@@ -184,6 +184,7 @@ class FontLoader {
         supported = true;
       } else if (
         typeof navigator !== "undefined" &&
+        typeof navigator?.userAgent === "string" &&
         // User agent string sniffing is bad, but there is no reliable way to
         // tell if the font is fully loaded and ready to be used with canvas.
         /Mozilla\/5.0.*?rv:\d+.*? Gecko/.test(navigator.userAgent)
diff --git a/src/shared/util.js b/src/shared/util.js
index ed1ec6886..5a4874ffe 100644
--- a/src/shared/util.js
+++ b/src/shared/util.js
@@ -616,14 +616,15 @@ class FeatureTest {
 
   static get platform() {
     if (
-      (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) &&
-      typeof navigator === "undefined"
+      (typeof PDFJSDev !== "undefined" && PDFJSDev.test("MOZCENTRAL")) ||
+      (typeof navigator !== "undefined" &&
+        typeof navigator?.platform === "string")
     ) {
-      return shadow(this, "platform", { isMac: false });
+      return shadow(this, "platform", {
+        isMac: navigator.platform.includes("Mac"),
+      });
     }
-    return shadow(this, "platform", {
-      isMac: navigator.platform.includes("Mac"),
-    });
+    return shadow(this, "platform", { isMac: false });
   }
 
   static get isCSSRoundSupported() {