diff --git a/dom/xhr/XMLHttpRequestMainThread.cpp b/dom/xhr/XMLHttpRequestMainThread.cpp
index 110ff3202623..5dc99de52bb8 100644
--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -2776,16 +2776,19 @@ nsresult XMLHttpRequestMainThread::SendInternal(const BodyExtractorBase* aBody,
         uploadContentType = defaultContentType;
       } else if (aBodyIsDocumentOrString &&
                  StaticPrefs::dom_xhr_standard_content_type_normalization()) {
-        UniquePtr<CMimeType> parsed = CMimeType::Parse(uploadContentType);
-        if (parsed && parsed->HasParameter(kLiteralString_charset)) {
-          parsed->SetParameterValue(kLiteralString_charset,
-                                    kLiteralString_UTF_8);
-          parsed->Serialize(uploadContentType);
+        UniquePtr<CMimeType> contentTypeRecord =
+            CMimeType::Parse(uploadContentType);
+        nsAutoCString charset;
+        if (contentTypeRecord &&
+            contentTypeRecord->GetParameterValue(kLiteralString_charset,
+                                                 charset) &&
+            !charset.EqualsIgnoreCase("utf-8")) {
+          contentTypeRecord->SetParameterValue(kLiteralString_charset,
+                                               kLiteralString_UTF_8);
+          contentTypeRecord->Serialize(uploadContentType);
         }
-      }
-
-      // We don't want to set a charset for streams.
-      if (!charset.IsEmpty()) {
+      } else if (!charset.IsEmpty()) {
+        // We don't want to set a charset for streams.
         // Replace all case-insensitive matches of the charset in the
         // content-type with the correct case.
         RequestHeaders::CharsetIterator iter(uploadContentType);
diff --git a/dom/xhr/tests/test_XHRSendData.html b/dom/xhr/tests/test_XHRSendData.html
index 653e3e5ca17e..78aa72548276 100644
--- a/dom/xhr/tests/test_XHRSendData.html
+++ b/dom/xhr/tests/test_XHRSendData.html
@@ -124,7 +124,7 @@ tests = [{ body: null,
          { body: "hi",
            contentType: "foo/bar; charset=uTf-8",
            resBody: "hi",
-           resContentType: "foo/bar;charset=UTF-8",
+           resContentType: "foo/bar; charset=uTf-8",
          },
          { body: testDoc1,
            resBody: "<!-- comment -->\n<out>hi</out>",
@@ -162,7 +162,7 @@ tests = [{ body: null,
          { body: testDoc2,
            contentType: "foo/bar; charset=uTf-8",
            resBody: "<!-- doc 2 -->\n<res>text</res>",
-           resContentType: "foo/bar;charset=UTF-8",
+           resContentType: "foo/bar; charset=uTf-8",
          },
          { //will trigger a redirect test server-side
            body: ("TEST_REDIRECT_STR&url=" + window.location.host + window.location.pathname),
diff --git a/modules/libpref/init/StaticPrefList.h b/modules/libpref/init/StaticPrefList.h
index 02da4fac5d84..6804e1567232 100644
--- a/modules/libpref/init/StaticPrefList.h
+++ b/modules/libpref/init/StaticPrefList.h
@@ -483,11 +483,10 @@ VARCACHE_PREF(
 )
 
 // Enable content type normalization of XHR uploads via MIME Sniffing standard
-// Disabled for now in bz1499136
 VARCACHE_PREF(
   "dom.xhr.standard_content_type_normalization",
    dom_xhr_standard_content_type_normalization,
-  RelaxedAtomicBool, false
+  RelaxedAtomicBool, true
 )
 
 // Block multiple external protocol URLs in iframes per single event.
diff --git a/testing/web-platform/meta/xhr/send-content-type-charset.htm.ini b/testing/web-platform/meta/xhr/send-content-type-charset.htm.ini
deleted file mode 100644
index 5853a3796501..000000000000
--- a/testing/web-platform/meta/xhr/send-content-type-charset.htm.ini
+++ /dev/null
@@ -1,19 +0,0 @@
-[send-content-type-charset.htm]
-  [charset with space that is UTF-8 does not change]
-    expected: FAIL
-
-  [charset in double quotes with backslashes that is UTF-8 does not change]
-    expected: FAIL
-
-  [If charset= param is UTF-8 (case-insensitive), it should not be changed (bogus charset)]
-    expected: FAIL
-
-  [If charset= param is UTF-8 (case-insensitive), it should not be changed]
-    expected: FAIL
-
-  [charset in double quotes that is UTF-8 does not change]
-    expected: FAIL
-
-  [charset with trailing space that is UTF-8 does not change]
-    expected: FAIL
-
diff --git a/toolkit/modules/tests/xpcshell/test_Http.js b/toolkit/modules/tests/xpcshell/test_Http.js
index 88f1c8c0ce6e..b55aa6bd7092 100644
--- a/toolkit/modules/tests/xpcshell/test_Http.js
+++ b/toolkit/modules/tests/xpcshell/test_Http.js
@@ -18,7 +18,7 @@ const kPostPath = "/post";
 const kPostUrl = kBaseUrl + kPostPath;
 const kPostDataSent = [["foo", "bar"], ["complex", "!*()@"]];
 const kPostDataReceived = "foo=bar&complex=%21%2A%28%29%40";
-const kPostMimeTypeReceived = "application/x-www-form-urlencoded;charset=UTF-8";
+const kPostMimeTypeReceived = "application/x-www-form-urlencoded; charset=utf-8";
 
 const kJsonPostPath = "/json_post";
 const kJsonPostUrl = kBaseUrl + kJsonPostPath;