diff --git a/netwerk/protocol/http/nsHttpHandler.cpp b/netwerk/protocol/http/nsHttpHandler.cpp
index a3a6d312a447..1ef14cdf1bc7 100644
--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -1519,6 +1519,25 @@ nsHttpHandler::TimerCallback(nsITimer * aTimer, void * aClosure)
         thisObject->mCapabilities &= ~NS_HTTP_ALLOW_PIPELINING;
 }
 
+static void
+NormalizeLanguageTag(char *code)
+{
+    bool is_region = false;
+    while (*code != '\0')
+    {
+        if (*code == '-') {
+            is_region = true;
+        } else {
+            if (is_region) {
+                *code = nsCRT::ToUpper(*code);
+            } else {
+                *code = nsCRT::ToLower(*code);
+            }
+        }
+        code++;
+    }
+}
+
 /**
  *  Allocates a C string into that contains a ISO 639 language list
  *  notated with HTTP "q" values for output with a HTTP Accept-Language
@@ -1574,6 +1593,8 @@ PrepareAcceptLanguages(const char *i_AcceptLanguages, nsACString &o_AcceptLangua
             *trim = '\0';
 
         if (*token != '\0') {
+            NormalizeLanguageTag(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
new file mode 100644
index 000000000000..c13494dbb7af
--- /dev/null
+++ b/netwerk/test/unit/test_header_Accept-Language_case.js
@@ -0,0 +1,32 @@
+var testpath = "/bug1054739";
+
+function run_test() {
+  let intlPrefs = Cc["@mozilla.org/preferences-service;1"].getService(Ci.nsIPrefService).getBranch("intl.");
+
+  let oldAcceptLangPref = intlPrefs.getCharPref("accept_languages");
+
+  let testData = [
+    ["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"],
+    ["EN-US;q=0.2, EN", "en-US,en;q=0.5"],
+  ];
+
+  for (let i = 0; i < testData.length; i++) {
+    let acceptLangPref = testData[i][0];
+    let expectedHeader = testData[i][1];
+
+    intlPrefs.setCharPref("accept_languages", acceptLangPref);
+    let acceptLangHeader = setupChannel(testpath).getRequestHeader("Accept-Language");
+    equal(acceptLangHeader, expectedHeader);
+  }
+
+  intlPrefs.setCharPref("accept_languages", oldAcceptLangPref);
+}
+
+function setupChannel(path) {
+  let ios = Cc["@mozilla.org/network/io-service;1"].getService(Ci.nsIIOService);
+  let chan = ios.newChannel("http://localhost:4444" + path, "", null);
+  chan.QueryInterface(Ci.nsIHttpChannel);
+  return chan;
+}
diff --git a/netwerk/test/unit/xpcshell.ini b/netwerk/test/unit/xpcshell.ini
index 1510577028b4..54df63810940 100644
--- a/netwerk/test/unit/xpcshell.ini
+++ b/netwerk/test/unit/xpcshell.ini
@@ -198,6 +198,7 @@ skip-if = bits != 32
 [test_gzipped_206.js]
 [test_head.js]
 [test_header_Accept-Language.js]
+[test_header_Accept-Language_case.js]
 [test_headers.js]
 [test_http_headers.js]
 [test_httpauth.js]