diff --git a/netwerk/protocol/http/nsHttpHandler.cpp b/netwerk/protocol/http/nsHttpHandler.cpp
index ed91f49584d2..9b7e9cfa2caf 100644
--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -1562,22 +1562,47 @@ nsHttpHandler::TimerCallback(nsITimer * aTimer, void * aClosure)
         thisObject->mCapabilities &= ~NS_HTTP_ALLOW_PIPELINING;
 }
 
+/**
+ * Currently, only regularizes the case of subtags.
+ */
 static void
-NormalizeLanguageTag(char *code)
+CanonicalizeLanguageTag(char *languageTag)
 {
-    bool is_region = false;
-    while (*code != '\0')
-    {
-        if (*code == '-') {
-            is_region = true;
+    char *s = languageTag;
+    while (*s != '\0') {
+        *s = nsCRT::ToLower(*s);
+        s++;
+    }
+
+    s = languageTag;
+    bool isFirst = true;
+    bool seenSingleton = false;
+    while (*s != '\0') {
+        char *subTagEnd = strchr(s, '-');
+        if (subTagEnd == nullptr) {
+            subTagEnd = strchr(s, '\0');
+        }
+
+        if (isFirst) {
+            isFirst = false;
+        } else if (seenSingleton) {
+            // Do nothing
         } else {
-            if (is_region) {
-                *code = nsCRT::ToUpper(*code);
-            } else {
-                *code = nsCRT::ToLower(*code);
+            size_t subTagLength = subTagEnd - s;
+            if (subTagLength == 1) {
+                seenSingleton = true;
+            } else if (subTagLength == 2) {
+                *s = nsCRT::ToUpper(*s);
+                *(s + 1) = nsCRT::ToUpper(*(s + 1));
+            } else if (subTagLength == 4) {
+                *s = nsCRT::ToUpper(*s);
             }
         }
-        code++;
+
+        s = subTagEnd;
+        if (*s != '\0') {
+            s++;
+        }
     }
 }
 
@@ -1636,7 +1661,7 @@ PrepareAcceptLanguages(const char *i_AcceptLanguages, nsACString &o_AcceptLangua
             *trim = '\0';
 
         if (*token != '\0') {
-            NormalizeLanguageTag(token);
+            CanonicalizeLanguageTag(token);
 
             comma = count_n++ != 0 ? "," : ""; // delimiter if not first item
             uint32_t u = QVAL_TO_UINT(q);
diff --git a/netwerk/test/unit/test_header_Accept-Language_case.js b/netwerk/test/unit/test_header_Accept-Language_case.js
index f64e5cffaff7..2374ff1f8b72 100644
--- a/netwerk/test/unit/test_header_Accept-Language_case.js
+++ b/netwerk/test/unit/test_header_Accept-Language_case.js
@@ -8,6 +8,16 @@ function run_test() {
   let oldAcceptLangPref = intlPrefs.getCharPref("accept_languages");
 
   let testData = [
+    ["en",              "en"],
+    ["ast",             "ast"],
+    ["fr-ca",           "fr-CA"],
+    ["zh-yue",          "zh-yue"],
+    ["az-latn",         "az-Latn"],
+    ["sl-nedis",        "sl-nedis"],
+    ["zh-hant-hk",      "zh-Hant-HK"],
+    ["ZH-HANT-HK",      "zh-Hant-HK"],
+    ["en-us-x-priv",    "en-US-x-priv"],
+    ["en-us-x-twain",   "en-US-x-twain"],
     ["de, en-US, en",   "de,en-US;q=0.7,en;q=0.3"],
     ["de,en-us,en",     "de,en-US;q=0.7,en;q=0.3"],
     ["en-US, en",       "en-US,en;q=0.5"],