diff --git a/intl/components/src/IDNA.h b/intl/components/src/IDNA.h
index f7af2f245422..9f186614032f 100644
--- a/intl/components/src/IDNA.h
+++ b/intl/components/src/IDNA.h
@@ -65,6 +65,15 @@ class IDNA final {
       return (mErrorCode & UIDNA_ERROR_PUNYCODE) != 0;
     }
 
+    /* The label was successfully ACE (Punycode) decoded but the resulting
+     * string had severe validation errors. For example,
+     * it might contain characters that are not allowed in ACE labels,
+     * or it might not be normalized.
+     */
+    bool HasInvalidAceLabel() const {
+      return (mErrorCode & UIDNA_ERROR_INVALID_ACE_LABEL) != 0;
+    }
+
     /**
      * Checks if the domain name label has any invalid hyphen characters.
      *
diff --git a/netwerk/dns/nsIDNService.cpp b/netwerk/dns/nsIDNService.cpp
index 9ec981eaf173..52401cbc78a3 100644
--- a/netwerk/dns/nsIDNService.cpp
+++ b/netwerk/dns/nsIDNService.cpp
@@ -188,8 +188,8 @@ nsresult nsIDNService::IDNA2008StringPrep(const nsAString& input,
   // appears to get an appended U+FFFD REPLACEMENT CHARACTER, which will
   // confuse our subsequent processing, so we drop that.
   // (https://bugzilla.mozilla.org/show_bug.cgi?id=1399540#c9)
-  if (info.HasInvalidPunycode() && !output.IsEmpty() &&
-      output.Last() == 0xfffd) {
+  if ((info.HasInvalidPunycode() || info.HasInvalidAceLabel()) &&
+      !output.IsEmpty() && output.Last() == 0xfffd) {
     output.Truncate(output.Length() - 1);
   }
 
diff --git a/netwerk/test/unit/test_URIs.js b/netwerk/test/unit/test_URIs.js
index 09484b43270c..622f7199db6e 100644
--- a/netwerk/test/unit/test_URIs.js
+++ b/netwerk/test/unit/test_URIs.js
@@ -1036,3 +1036,8 @@ add_task(function test_iconURI_serialization() {
 add_task(function test_jarURI_serialization() {
   check_round_trip_serialization("jar:http://example.com/bar.jar!/");
 });
+
+add_task(async function round_trip_invalid_ace_label() {
+  let uri = Services.io.newURI("http://xn--xn--d--fg4n-5y45d/");
+  Assert.equal(uri.spec, "http://xn--xn--d--fg4n-5y45d/");
+});