--- a/netwerk/base/nsStandardURL.cpp
+++ b/netwerk/base/nsStandardURL.cpp
@@ -1,16 +1,17 @@
 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
 /* vim:set ts=4 sw=4 sts=4 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "IPCMessageUtils.h"
 
+#include "nsASCIIMask.h"
 #include "nsStandardURL.h"
 #include "nsCRT.h"
 #include "nsEscape.h"
 #include "nsIFile.h"
 #include "nsIObjectInputStream.h"
 #include "nsIObjectOutputStream.h"
 #include "nsIPrefService.h"
 #include "nsIPrefBranch.h"
@@ -137,16 +138,35 @@ namespace net {
 
 static NS_DEFINE_CID(kThisImplCID, NS_THIS_STANDARDURL_IMPL_CID);
 static NS_DEFINE_CID(kStandardURLCID, NS_STANDARDURL_CID);
 
 nsIIDNService *nsStandardURL::gIDN = nullptr;
 bool nsStandardURL::gInitialized = false;
 char nsStandardURL::gHostLimitDigits[] = { '/', '\\', '?', '#', 0 };
 
+// Invalid host characters
+// We still allow % because it is in the ID of addons.
+// Any percent encoded ASCII characters that are not allowed in the
+// hostname are not percent decoded, and will be parsed just fine.
+//
+// Note that the array below will be initialized at compile time,
+// so we do not need to "optimize" TestForInvalidHostCharacters.
+//
+constexpr bool TestForInvalidHostCharacters(char c)
+{
+    // Testing for these:
+    // CONTROL_CHARACTERS " #/:?@[\\]*<>|\"";
+    return (c > 0 && c < 32) || // The control characters are [1, 31]
+           c == ' ' || c == '#' || c == '/' || c == ':' || c == '?' ||
+           c == '@' || c == '[' || c == '\\' || c == ']' || c == '*' ||
+           c == '<' || c == '>' || c == '|' || c == '"';
+}
+constexpr ASCIIMaskArray sInvalidHostChars = CreateASCIIMask(TestForInvalidHostCharacters);
+
 //----------------------------------------------------------------------------
 
 #define ENSURE_MUTABLE() \
   PR_BEGIN_MACRO \
     if (!mMutable) { \
         NS_WARNING("attempt to modify an immutable nsStandardURL"); \
         return NS_ERROR_ABORT; \
     } \
@@ -685,24 +705,23 @@ nsStandardURL::ValidIPv6orHostname(const
         return net_IsValidIPv6Addr(host + 1, length - 2);
     }
 
     if (openBracket || closeBracket) {
         // Fail if only one of the brackets is present
         return false;
     }
 
-    const char *end = host + length;
-    if (end != net_FindCharInSet(host, end, CONTROL_CHARACTERS " #/:?@[\\]*<>|\"")) {
-        // We still allow % because it is in the ID of addons.
-        // Any percent encoded ASCII characters that are not allowed in the
-        // hostname are not percent decoded, and will be parsed just fine.
-        return false;
+    const char* end = host + length;
+    const char* iter = host;
+    for (; iter != end && *iter; ++iter) {
+        if (ASCIIMask::IsMasked(sInvalidHostChars, *iter)) {
+            return false;
+        }
     }
-
     return true;
 }
 
 void
 nsStandardURL::CoalescePath(netCoalesceFlags coalesceFlag, char *path)
 {
     net_CoalesceDirs(coalesceFlag, path);
     int32_t newLen = strlen(path);