diff --git a/browser/base/content/test/general/file_trackingUI_6.html b/browser/base/content/test/general/file_trackingUI_6.html
index a0fd3054b4f1..52e1ae63ffd7 100644
--- a/browser/base/content/test/general/file_trackingUI_6.html
+++ b/browser/base/content/test/general/file_trackingUI_6.html
@@ -6,7 +6,7 @@
 </head>
 <body>
   <p>Hello there!</p>
-  <script type="application/javascript">
+  <script type="application/javascript; version=1.8">
     function test_fetch() {
       let url = "http://trackertest.org/browser/browser/base/content/test/general/file_trackingUI_6.js";
       return fetch(url);
diff --git a/devtools/client/shared/components/test/mochitest/test_HSplitBox_01.html b/devtools/client/shared/components/test/mochitest/test_HSplitBox_01.html
index d65bbe70bebb..e717fca20be6 100644
--- a/devtools/client/shared/components/test/mochitest/test_HSplitBox_01.html
+++ b/devtools/client/shared/components/test/mochitest/test_HSplitBox_01.html
@@ -10,7 +10,7 @@ Basic tests for the HSplitBox component.
   <meta charset="utf-8">
   <title>Tree component test</title>
   <script type="application/javascript" src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
-  <script type="application/javascript" src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"></script>
+  <script type="application/javascript "src="chrome://mochikit/content/tests/SimpleTest/EventUtils.js"></script>
   <link rel="stylesheet" type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css"/>
   <link rel="stylesheet" href="resource://devtools/client/themes/splitters.css" type="text/css"/>
   <link rel="stylesheet" href="chrome://devtools/skin/components-h-split-box.css" type="text/css"/>
diff --git a/devtools/client/storage/test/storage-cookies-samesite.html b/devtools/client/storage/test/storage-cookies-samesite.html
index c14e2c67a839..ca91e1fb59c3 100644
--- a/devtools/client/storage/test/storage-cookies-samesite.html
+++ b/devtools/client/storage/test/storage-cookies-samesite.html
@@ -5,7 +5,7 @@
     <title>Storage inspector cookie samesite test</title>
   </head>
   <body>
-    <script type="application/javascript">
+    <script type="application/javascript;version=1.7">
     "use strict";
     let expiresIn24Hours = new Date(Date.now() + 60 * 60 * 24 * 1000).toUTCString();
 
diff --git a/dom/browser-element/mochitest/test_browserElement_inproc_ExposableURI.html b/dom/browser-element/mochitest/test_browserElement_inproc_ExposableURI.html
index 8e0bc2894ba8..089e5db4427d 100644
--- a/dom/browser-element/mochitest/test_browserElement_inproc_ExposableURI.html
+++ b/dom/browser-element/mochitest/test_browserElement_inproc_ExposableURI.html
@@ -12,7 +12,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=795317
 <body>
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=795317">Mozilla Bug 795317</a>
 
-<script type="application/javascript" src="browserElement_ExposableURI.js"></script>
+<script type="application/javascript;version1.7" src="browserElement_ExposableURI.js"></script>
 
 </body>
 </html>
diff --git a/dom/browser-element/mochitest/test_browserElement_inproc_LoadEvents.html b/dom/browser-element/mochitest/test_browserElement_inproc_LoadEvents.html
index 7f4a296c632f..3370ff87419f 100644
--- a/dom/browser-element/mochitest/test_browserElement_inproc_LoadEvents.html
+++ b/dom/browser-element/mochitest/test_browserElement_inproc_LoadEvents.html
@@ -12,7 +12,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=710231
 <body>
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=710231">Mozilla Bug 710231</a>
 
-<script type="application/javascript" src="browserElement_LoadEvents.js"></script>
+<script type="application/javascript;version1.7" src="browserElement_LoadEvents.js"></script>
 
 </body>
 </html>
diff --git a/dom/browser-element/mochitest/test_browserElement_oop_ExposableURI.html b/dom/browser-element/mochitest/test_browserElement_oop_ExposableURI.html
index 8e0bc2894ba8..089e5db4427d 100644
--- a/dom/browser-element/mochitest/test_browserElement_oop_ExposableURI.html
+++ b/dom/browser-element/mochitest/test_browserElement_oop_ExposableURI.html
@@ -12,7 +12,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=795317
 <body>
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=795317">Mozilla Bug 795317</a>
 
-<script type="application/javascript" src="browserElement_ExposableURI.js"></script>
+<script type="application/javascript;version1.7" src="browserElement_ExposableURI.js"></script>
 
 </body>
 </html>
diff --git a/dom/browser-element/mochitest/test_browserElement_oop_LoadEvents.html b/dom/browser-element/mochitest/test_browserElement_oop_LoadEvents.html
index 2cb3c30bdedc..c6daf805e1b7 100644
--- a/dom/browser-element/mochitest/test_browserElement_oop_LoadEvents.html
+++ b/dom/browser-element/mochitest/test_browserElement_oop_LoadEvents.html
@@ -12,7 +12,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=710231
 <body>
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=710231">Mozilla Bug 710231</a>
 
-<script type="application/javascript" src="browserElement_LoadEvents.js"></script>
+<script type="application/javascript;version1.7" src="browserElement_LoadEvents.js"></script>
 
 </body>
 </html>
diff --git a/dom/html/HTMLScriptElement.cpp b/dom/html/HTMLScriptElement.cpp
index aa2fb9765260..28fd21572dac 100644
--- a/dom/html/HTMLScriptElement.cpp
+++ b/dom/html/HTMLScriptElement.cpp
@@ -171,20 +171,9 @@ HTMLScriptElement::SetText(const nsAString& aValue, ErrorResult& aRv)
 // need to be transfered when modifying
 
 bool
-HTMLScriptElement::GetScriptType(nsAString& aType)
+HTMLScriptElement::GetScriptType(nsAString& type)
 {
-  nsAutoString type;
-  if (!GetAttr(kNameSpaceID_None, nsGkAtoms::type, type)) {
-    return false;
-  }
-
-  // ASCII whitespace https://infra.spec.whatwg.org/#ascii-whitespace:
-  // U+0009 TAB, U+000A LF, U+000C FF, U+000D CR, or U+0020 SPACE.
-  static const char kASCIIWhitespace[] = "\t\n\f\r ";
-  type.Trim(kASCIIWhitespace);
-
-  aType.Assign(type);
-  return true;
+  return GetAttr(kNameSpaceID_None, nsGkAtoms::type, type);
 }
 
 void
diff --git a/dom/html/test/test_checked.html b/dom/html/test/test_checked.html
index eb021ea3df08..a445106234f7 100644
--- a/dom/html/test/test_checked.html
+++ b/dom/html/test/test_checked.html
@@ -26,7 +26,7 @@ and
   </menu>
 </div>
 <pre id="test">
-<script class="testbody" type="text/javascript">
+<script class="testbody" type="text/javascript; version=1.7">
 
 /** Test for Bug 418756 and 617528 **/
 var group1;
diff --git a/dom/script/ModuleLoadRequest.cpp b/dom/script/ModuleLoadRequest.cpp
index b265e042cf08..10eddb889d3b 100644
--- a/dom/script/ModuleLoadRequest.cpp
+++ b/dom/script/ModuleLoadRequest.cpp
@@ -29,6 +29,7 @@ NS_IMPL_RELEASE_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
 
 ModuleLoadRequest::ModuleLoadRequest(nsIURI* aURI,
                                      nsIScriptElement* aElement,
+                                     ValidJSVersion aValidJSVersion,
                                      CORSMode aCORSMode,
                                      const SRIMetadata& aIntegrity,
                                      nsIURI* aReferrer,
@@ -37,6 +38,7 @@ ModuleLoadRequest::ModuleLoadRequest(nsIURI* aURI,
   : ScriptLoadRequest(ScriptKind::eModule,
                       aURI,
                       aElement,
+                      aValidJSVersion,
                       aCORSMode,
                       aIntegrity,
                       aReferrer,
@@ -53,6 +55,7 @@ ModuleLoadRequest::ModuleLoadRequest(nsIURI* aURI,
   : ScriptLoadRequest(ScriptKind::eModule,
                       aURI,
                       aParent->mElement,
+                      aParent->mValidJSVersion,
                       aParent->mCORSMode,
                       SRIMetadata(),
                       aParent->mURI,
diff --git a/dom/script/ModuleLoadRequest.h b/dom/script/ModuleLoadRequest.h
index b70619a25976..e835f1967110 100644
--- a/dom/script/ModuleLoadRequest.h
+++ b/dom/script/ModuleLoadRequest.h
@@ -45,6 +45,7 @@ public:
   // Create a top-level module load request.
   ModuleLoadRequest(nsIURI* aURI,
                     nsIScriptElement* aElement,
+                    ValidJSVersion aValidJSVersion,
                     CORSMode aCORSMode,
                     const SRIMetadata& aIntegrity,
                     nsIURI* aReferrer,
diff --git a/dom/script/ScriptLoadRequest.cpp b/dom/script/ScriptLoadRequest.cpp
index 2c8856a21358..a7fd6f09eff2 100644
--- a/dom/script/ScriptLoadRequest.cpp
+++ b/dom/script/ScriptLoadRequest.cpp
@@ -41,6 +41,7 @@ NS_IMPL_CYCLE_COLLECTION_TRACE_END
 ScriptLoadRequest::ScriptLoadRequest(ScriptKind aKind,
                                      nsIURI* aURI,
                                      nsIScriptElement* aElement,
+                                     ValidJSVersion aValidJSVersion,
                                      mozilla::CORSMode aCORSMode,
                                      const SRIMetadata& aIntegrity,
                                      nsIURI* aReferrer,
@@ -64,6 +65,7 @@ ScriptLoadRequest::ScriptLoadRequest(ScriptKind aKind,
   , mScriptText()
   , mScriptBytecode()
   , mBytecodeOffset(0)
+  , mValidJSVersion(aValidJSVersion)
   , mURI(aURI)
   , mLineNo(1)
   , mCORSMode(aCORSMode)
diff --git a/dom/script/ScriptLoadRequest.h b/dom/script/ScriptLoadRequest.h
index ec6ade8f8cd8..8deead6083d8 100644
--- a/dom/script/ScriptLoadRequest.h
+++ b/dom/script/ScriptLoadRequest.h
@@ -29,6 +29,11 @@ enum class ScriptKind {
   eModule
 };
 
+enum class ValidJSVersion : bool {
+  eInvalid,
+  eValid
+};
+
 /*
  * A class that handles loading and evaluation of <script> elements.
  */
@@ -49,6 +54,7 @@ public:
   ScriptLoadRequest(ScriptKind aKind,
                     nsIURI* aURI,
                     nsIScriptElement* aElement,
+                    ValidJSVersion aValidJSVersion,
                     mozilla::CORSMode aCORSMode,
                     const SRIMetadata &aIntegrity,
                     nsIURI* aReferrer,
@@ -211,6 +217,7 @@ public:
   mozilla::Vector<uint8_t> mScriptBytecode;
   uint32_t mBytecodeOffset; // Offset of the bytecode in mScriptBytecode
 
+  ValidJSVersion mValidJSVersion;
   const nsCOMPtr<nsIURI> mURI;
   nsCOMPtr<nsIPrincipal> mTriggeringPrincipal;
   nsCOMPtr<nsIPrincipal> mOriginPrincipal;
diff --git a/dom/script/ScriptLoader.cpp b/dom/script/ScriptLoader.cpp
index ce49f528f6e9..e0b36fb223bd 100644
--- a/dom/script/ScriptLoader.cpp
+++ b/dom/script/ScriptLoader.cpp
@@ -1059,11 +1059,12 @@ ScriptLoader::StartLoad(ScriptLoadRequest* aRequest)
 
   NS_ENSURE_SUCCESS(rv, rv);
 
-  // To avoid decoding issues, the build-id is part of the JSBytecodeMimeType
-  // constant.
+  // To avoid decoding issues, the JSVersion is explicitly guarded here, and the
+  // build-id is part of the JSBytecodeMimeType constant.
   aRequest->mCacheInfo = nullptr;
   nsCOMPtr<nsICacheInfoChannel> cic(do_QueryInterface(channel));
-  if (cic && nsContentUtils::IsBytecodeCacheEnabled()) {
+  if (cic && nsContentUtils::IsBytecodeCacheEnabled() &&
+      aRequest->mValidJSVersion == ValidJSVersion::eValid) {
     if (!aRequest->IsLoadingSource()) {
       // Inform the HTTP cache that we prefer to have information coming from the
       // bytecode cache instead of the sources, if such entry is already registered.
@@ -1173,6 +1174,63 @@ ScriptLoader::PreloadURIComparator::Equals(const PreloadInfo& aPi,
          same;
 }
 
+/**
+ * Returns ValidJSVersion::eValid if aVersionStr is a string of the form '1.n',
+ * n = 0, ..., 8, and ValidJSVersion::eInvalid for other strings.
+ */
+static ValidJSVersion
+ParseJavascriptVersion(const nsAString& aVersionStr)
+{
+  if (aVersionStr.Length() != 3 || aVersionStr[0] != '1' ||
+      aVersionStr[1] != '.') {
+    return ValidJSVersion::eInvalid;
+  }
+  if ('0' <= aVersionStr[2] && aVersionStr[2] <= '8') {
+    return ValidJSVersion::eValid;
+  }
+  return ValidJSVersion::eInvalid;
+}
+
+static inline bool
+ParseTypeAttribute(const nsAString& aType, ValidJSVersion* aVersion)
+{
+  MOZ_ASSERT(!aType.IsEmpty());
+  MOZ_ASSERT(aVersion);
+  MOZ_ASSERT(*aVersion == ValidJSVersion::eValid);
+
+  nsContentTypeParser parser(aType);
+
+  nsAutoString mimeType;
+  nsresult rv = parser.GetType(mimeType);
+  NS_ENSURE_SUCCESS(rv, false);
+
+  if (!nsContentUtils::IsJavascriptMIMEType(mimeType)) {
+    return false;
+  }
+
+  // Get the version string, and ensure the language supports it.
+  nsAutoString versionName;
+  rv = parser.GetParameter("version", versionName);
+
+  if (rv == NS_ERROR_INVALID_ARG) {
+    Telemetry::Accumulate(Telemetry::SCRIPT_LOADED_WITH_VERSION, false);
+    // Argument not set.
+    return true;
+  }
+
+  if (NS_FAILED(rv)) {
+    return false;
+  }
+
+  *aVersion = ParseJavascriptVersion(versionName);
+  if (*aVersion == ValidJSVersion::eValid) {
+    Telemetry::Accumulate(Telemetry::SCRIPT_LOADED_WITH_VERSION, true);
+    return true;
+  }
+
+  return true;
+}
+
 static bool
 CSPAllowsInlineScript(nsIScriptElement* aElement, nsIDocument* aDocument)
 {
@@ -1205,6 +1263,7 @@ ScriptLoadRequest*
 ScriptLoader::CreateLoadRequest(ScriptKind aKind,
                                 nsIURI* aURI,
                                 nsIScriptElement* aElement,
+                                ValidJSVersion aValidJSVersion,
                                 CORSMode aCORSMode,
                                 const SRIMetadata& aIntegrity,
                                 mozilla::net::ReferrerPolicy aReferrerPolicy)
@@ -1213,7 +1272,7 @@ ScriptLoader::CreateLoadRequest(ScriptKind aKind,
 
   if (aKind == ScriptKind::eClassic) {
     ScriptLoadRequest* slr = new ScriptLoadRequest(aKind, aURI, aElement,
-                                                   aCORSMode, aIntegrity,
+                                                   aValidJSVersion, aCORSMode, aIntegrity,
                                                    referrer, aReferrerPolicy);
 
     LOG(("ScriptLoader %p creates ScriptLoadRequest %p", this, slr));
@@ -1221,8 +1280,8 @@ ScriptLoader::CreateLoadRequest(ScriptKind aKind,
   }
 
   MOZ_ASSERT(aKind == ScriptKind::eModule);
-  return new ModuleLoadRequest(aURI, aElement, aCORSMode, aIntegrity, referrer,
-                               aReferrerPolicy, this);
+  return new ModuleLoadRequest(aURI, aElement, aValidJSVersion, aCORSMode,
+                               aIntegrity, referrer, aReferrerPolicy, this);
 }
 
 bool
@@ -1251,11 +1310,13 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
     return false;
   }
 
+  ValidJSVersion validJSVersion = ValidJSVersion::eValid;
+
   // For classic scripts, check the type attribute to determine language and
   // version. If type exists, it trumps the deprecated 'language='
   if (scriptKind == ScriptKind::eClassic) {
     if (!type.IsEmpty()) {
-      NS_ENSURE_TRUE(nsContentUtils::IsJavascriptMIMEType(type), false);
+      NS_ENSURE_TRUE(ParseTypeAttribute(type, &validJSVersion), false);
     } else if (!hasType) {
       // no 'type=' element
       // "language" is a deprecated attribute of HTML, so we check it only for
@@ -1368,8 +1429,9 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
         principal = scriptContent->NodePrincipal();
       }
 
-      request = CreateLoadRequest(scriptKind, scriptURI, aElement, ourCORSMode,
-                                  sriMetadata, ourRefPolicy);
+      request = CreateLoadRequest(scriptKind, scriptURI, aElement,
+                                  validJSVersion, ourCORSMode, sriMetadata,
+                                  ourRefPolicy);
       request->mTriggeringPrincipal = Move(principal);
       request->mIsInline = false;
       request->SetScriptMode(aElement->GetScriptDeferred(),
@@ -1394,6 +1456,8 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
     NS_ASSERTION(!request->InCompilingStage(),
                  "Request should not yet be in compiling stage.");
 
+    request->mValidJSVersion = validJSVersion;
+
     if (request->IsAsyncScript()) {
       AddAsyncRequest(request);
       if (request->IsReadyToRun()) {
@@ -1496,9 +1560,10 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
   }
 
   request = CreateLoadRequest(scriptKind, mDocument->GetDocumentURI(), aElement,
-                              corsMode,
+                              validJSVersion, corsMode,
                               SRIMetadata(), // SRI doesn't apply
                               ourRefPolicy);
+  request->mValidJSVersion = validJSVersion;
   request->mIsInline = true;
   request->mTriggeringPrincipal = mDocument->NodePrincipal();
   request->mLineNo = aElement->GetScriptLineNumber();
@@ -2127,6 +2192,10 @@ ScriptLoader::EvaluateScript(ScriptLoadRequest* aRequest)
     return NS_ERROR_FAILURE;
   }
 
+  if (aRequest->mValidJSVersion == ValidJSVersion::eInvalid) {
+    return NS_OK;
+  }
+
   // New script entry point required, due to the "Create a script" sub-step of
   // http://www.whatwg.org/specs/web-apps/current-work/#execute-the-script-block
   nsAutoMicroTask mt;
@@ -3141,7 +3210,7 @@ ScriptLoader::PreloadURI(nsIURI* aURI, const nsAString& aCharset,
   }
 
   RefPtr<ScriptLoadRequest> request =
-    CreateLoadRequest(ScriptKind::eClassic, aURI, nullptr,
+    CreateLoadRequest(ScriptKind::eClassic, aURI, nullptr, ValidJSVersion::eValid,
                       Element::StringToCORSMode(aCrossOrigin), sriMetadata,
                       aReferrerPolicy);
   request->mTriggeringPrincipal = mDocument->NodePrincipal();
diff --git a/dom/script/ScriptLoader.h b/dom/script/ScriptLoader.h
index 0aafa9a63003..9ffb6e0a036a 100644
--- a/dom/script/ScriptLoader.h
+++ b/dom/script/ScriptLoader.h
@@ -342,6 +342,7 @@ private:
   ScriptLoadRequest* CreateLoadRequest(ScriptKind aKind,
                                        nsIURI* aURI,
                                        nsIScriptElement* aElement,
+                                       ValidJSVersion aValidJSVersion,
                                        mozilla::CORSMode aCORSMode,
                                        const SRIMetadata& aIntegrity,
                                        mozilla::net::ReferrerPolicy aReferrerPolicy);
diff --git a/dom/tests/mochitest/bugs/test_bug1171215.html b/dom/tests/mochitest/bugs/test_bug1171215.html
index 6b4f59ed9eae..af0d3c6a2fef 100644
--- a/dom/tests/mochitest/bugs/test_bug1171215.html
+++ b/dom/tests/mochitest/bugs/test_bug1171215.html
@@ -10,7 +10,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1022869
   <script type="application/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
   <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
   <iframe src="about:blank"></iframe>
-  <script type="text/javascript">
+  <script type="text/javascript; version=1.8">
 
   var f = document.getElementsByTagName("iframe")[0];
 
diff --git a/dom/tests/mochitest/bugs/test_no_find_showDialog.html b/dom/tests/mochitest/bugs/test_no_find_showDialog.html
index e404c2976b03..fee670023d5a 100644
--- a/dom/tests/mochitest/bugs/test_no_find_showDialog.html
+++ b/dom/tests/mochitest/bugs/test_no_find_showDialog.html
@@ -7,7 +7,7 @@
   <script type="application/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
   <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
   <iframe src="about:blank"></iframe>
-  <script type="text/javascript">
+  <script type="text/javascript; version=1.8">
 
   function checkForFindDialog() {
     let chromeScript = SpecialPowers.loadChromeScript(_ => {
@@ -89,4 +89,4 @@
 <pre id="test">
 </pre>
 </body>
-</html>
+</html>
\ No newline at end of file
diff --git a/dom/tests/mochitest/script/mochitest.ini b/dom/tests/mochitest/script/mochitest.ini
index 55fca4f05ae6..04eee20823a8 100644
--- a/dom/tests/mochitest/script/mochitest.ini
+++ b/dom/tests/mochitest/script/mochitest.ini
@@ -4,4 +4,3 @@ support-files =
 
 [test_bug1053321.html]
 skip-if = os == 'android' # bug 1386644
-[test_whitespace.html]
diff --git a/dom/tests/mochitest/script/test_bug1053321.html b/dom/tests/mochitest/script/test_bug1053321.html
index b75c973d51c0..8b64e3ce6ee2 100644
--- a/dom/tests/mochitest/script/test_bug1053321.html
+++ b/dom/tests/mochitest/script/test_bug1053321.html
@@ -23,8 +23,8 @@ are unexpectedly blocked.
 
 <body>
   <script>
-    SimpleTest.waitForExplicitFinish();
-
+    // No need for an async test, we make it all before window.onload.
+    //
     // We can't test whether the two scripts have not been executed here, since
     // preloads of the two images below (that unblock the two tested <head>
     // scripts) may happen sooner than this script executes.
@@ -33,7 +33,6 @@ are unexpectedly blocked.
     });
     window.addEventListener("load", function() {
       ok(window.script_executed_async, "Async script executed before onload");
-      SimpleTest.finish();
     }, true);
   </script>
   <img src="file_blocked_script.sjs?unblock&defer"/>
diff --git a/dom/tests/mochitest/script/test_whitespace.html b/dom/tests/mochitest/script/test_whitespace.html
deleted file mode 100644
index b59a929f1598..000000000000
--- a/dom/tests/mochitest/script/test_whitespace.html
+++ /dev/null
@@ -1,31 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<head>
-  <title>Test for ScriptLoader and type with whitespaces</title>
-  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
-</head>
-<body>
-
-<script type="application/javascript">
-let passed = false;
-
-let tests = [
-  [ " application/javascript", true ],
-  [ "\tapplication/javascript\n\r \t", true ],
-];
-
-for (let i = 0; i < tests.length; ++i) {
-  passed = false;
-
-  let script = document.createElement('script');
-  script.setAttribute('type', tests[i][0]);
-  script.innerText = "passed = true;";
-  document.body.appendChild(script);
-
-  ok (passed, tests[i][1], "Test " + tests[i][0] + " passed");
-}
-</script>
-
-</body>
-</html>
diff --git a/dom/xbl/test/test_bug389322.xhtml b/dom/xbl/test/test_bug389322.xhtml
index c81e11143a5d..4b80346bd275 100644
--- a/dom/xbl/test/test_bug389322.xhtml
+++ b/dom/xbl/test/test_bug389322.xhtml
@@ -105,13 +105,21 @@ function report(testName, success) {
 }
 ]]>
 </script>
+<script type="text/javascript; version=1.7"><![CDATA[
+  try {
+    eval("let x = 1;");
+    var success = true;
+  }
+  catch (e) { success = false; }
+  report("HTML script tags with explicit version", success)
+]]></script>
 <script type="text/javascript"><![CDATA[
   try {
     eval("let x = 1;");
     var success = true;
   }
   catch (e) { success = false; }
-  is(success, true, "let should work in HTML script tags");
+  is(success, true, "let should work in versionless HTML script tags");
 ]]></script>
 </pre>
 </body>
diff --git a/layout/style/test/bug453896_iframe.html b/layout/style/test/bug453896_iframe.html
index 5a1f9e5b378c..bca68c7e7ac1 100644
--- a/layout/style/test/bug453896_iframe.html
+++ b/layout/style/test/bug453896_iframe.html
@@ -5,7 +5,7 @@
   <title>Bug 453896 Test middle frame</title>
   <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
   <meta http-equiv="Content-Style-Type" content="text/css">
-  <script type="application/javascript">
+  <script type="application/javascript; version=1.7">
 
 function run(test_window)
 {
diff --git a/layout/style/test/chrome/test_moz_document_rules.html b/layout/style/test/chrome/test_moz_document_rules.html
index 233c53bf3ddc..f2759c0850c3 100644
--- a/layout/style/test/chrome/test_moz_document_rules.html
+++ b/layout/style/test/chrome/test_moz_document_rules.html
@@ -10,7 +10,7 @@
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=398962">Mozilla Bug 398962</a>
 <iframe id="iframe" src="http://mochi.test:8888/tests/layout/style/test/chrome/moz_document_helper.html"></iframe>
 <pre id="test">
-<script type="application/javascript">
+<script type="application/javascript; version=1.8">
 
 var [gStyleSheetService, gIOService] = (function() {
     return [
diff --git a/layout/style/test/test_addSheet.html b/layout/style/test/test_addSheet.html
index 1d7aa070363b..3f44f19706d9 100644
--- a/layout/style/test/test_addSheet.html
+++ b/layout/style/test/test_addSheet.html
@@ -12,7 +12,7 @@
 <iframe id="iframe2" src="additional_sheets_helper.html"></iframe>
 
 <pre id="test">
-<script type="application/javascript">
+<script type="application/javascript; version=1.8">
 
 let gIOService = SpecialPowers.Cc["@mozilla.org/network/io-service;1"]
   .getService(SpecialPowers.Ci.nsIIOService);
diff --git a/layout/style/test/test_additional_sheets.html b/layout/style/test/test_additional_sheets.html
index 5c933a67c75e..9ca5fa7690cd 100644
--- a/layout/style/test/test_additional_sheets.html
+++ b/layout/style/test/test_additional_sheets.html
@@ -9,7 +9,7 @@
 <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=737003">Mozilla Bug 737003</a>
 <iframe id="iframe" src="additional_sheets_helper.html"></iframe>
 <pre id="test">
-<script type="application/javascript">
+<script type="application/javascript; version=1.8">
 
 var gIOService = SpecialPowers.Cc["@mozilla.org/network/io-service;1"]
   .getService(SpecialPowers.Ci.nsIIOService)
diff --git a/layout/style/test/test_computed_style_prefs.html b/layout/style/test/test_computed_style_prefs.html
index 2bb8e02f0ecf..163176237cae 100644
--- a/layout/style/test/test_computed_style_prefs.html
+++ b/layout/style/test/test_computed_style_prefs.html
@@ -12,7 +12,7 @@
   
 </div>
 <pre id="test">
-<script type="application/javascript">
+<script type="application/javascript; version=1.7">
 
 /** Test that preffed off properties do not appear in computed style **/
 
diff --git a/layout/style/test/test_media_queries_dynamic.html b/layout/style/test/test_media_queries_dynamic.html
index 8c7cfe270174..ac04d803a136 100644
--- a/layout/style/test/test_media_queries_dynamic.html
+++ b/layout/style/test/test_media_queries_dynamic.html
@@ -15,7 +15,7 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=473400
   
 </div>
 <pre id="test">
-<script class="testbody" type="application/javascript">
+<script class="testbody" type="application/javascript; version=1.7">
 
 /** Test for Bug 473400 **/
 
diff --git a/testing/web-platform/meta/html/semantics/scripting-1/the-script-element/script-type-and-language-js.html.ini b/testing/web-platform/meta/html/semantics/scripting-1/the-script-element/script-type-and-language-js.html.ini
index e63acbeaa4ee..05f435573769 100644
--- a/testing/web-platform/meta/html/semantics/scripting-1/the-script-element/script-type-and-language-js.html.ini
+++ b/testing/web-platform/meta/html/semantics/scripting-1/the-script-element/script-type-and-language-js.html.ini
@@ -1,5 +1,194 @@
 [script-type-and-language-js.html]
-  [Script shouldn't run with type=" "]
+  [Script should run with type="application/ecmascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="application/javascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="application/x-ecmascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="application/x-javascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="\\fapplication/ecmascript"]
+    expected: FAIL
+
+  [Script should run with type="\\fapplication/javascript"]
+    expected: FAIL
+
+  [Script should run with type="\\fapplication/x-ecmascript"]
+    expected: FAIL
+
+  [Script should run with type="\\fapplication/x-javascript"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/ecmascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/javascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/x-ecmascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/x-javascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/ecmascript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/javascript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/x-ecmascript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="application/x-javascript\\0foo"]
+    expected: FAIL
+
+  [Script should run with type="text/ecmascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/javascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/javascript1.0\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/javascript1.1\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/javascript1.2\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/javascript1.3\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/javascript1.4\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/javascript1.5\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/jscript\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/livescript\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/x-ecmascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="text/x-javascript\\f"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/ecmascript"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/javascript"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/javascript1.0"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/javascript1.1"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/javascript1.2"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/javascript1.3"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/javascript1.4"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/javascript1.5"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/jscript"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/livescript"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/x-ecmascript"]
+    expected: FAIL
+
+  [Script should run with type="\\ftext/x-javascript"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/ecmascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.0\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.1\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.2\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.3\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.4\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.5\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/jscript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/livescript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/x-ecmascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/x-javascript\\0"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/ecmascript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.0\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.1\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.2\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.3\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.4\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/javascript1.5\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/jscript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/livescript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/x-ecmascript\\0foo"]
+    expected: FAIL
+
+  [Script shouldn't run with type="text/x-javascript\\0foo"]
     expected: FAIL
 
   [Script should run with language="ecmascript"]
@@ -25,3 +214,4 @@
 
   [Script should run with language="X-JAVASCRIPT"]
     expected: FAIL
+
diff --git a/toolkit/components/extensions/test/mochitest/mochitest-common.ini b/toolkit/components/extensions/test/mochitest/mochitest-common.ini
index 58442745437f..6ad08e392408 100644
--- a/toolkit/components/extensions/test/mochitest/mochitest-common.ini
+++ b/toolkit/components/extensions/test/mochitest/mochitest-common.ini
@@ -109,6 +109,7 @@ skip-if = os == 'android' # unsupported.
 [test_ext_idle.html]
 [test_ext_inIncognitoContext_window.html]
 skip-if = os == 'android' # Android does not support multiple windows.
+[test_ext_jsversion.html]
 [test_ext_listener_proxies.html]
 [test_ext_new_tab_processType.html]
 [test_ext_notifications.html]
diff --git a/toolkit/components/extensions/test/mochitest/test_ext_jsversion.html b/toolkit/components/extensions/test/mochitest/test_ext_jsversion.html
new file mode 100644
index 000000000000..18f7252e96e5
--- /dev/null
+++ b/toolkit/components/extensions/test/mochitest/test_ext_jsversion.html
@@ -0,0 +1,71 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <title>Test for simple WebExtension</title>
+  <meta charset="utf-8">
+  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
+  <script type="text/javascript" src="/tests/SimpleTest/ExtensionTestUtils.js"></script>
+  <script type="text/javascript" src="head.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+
+<script type="text/javascript">
+"use strict";
+
+add_task(async function test_versioned_js() {
+  // We need to deal with escaping the close script tags.
+  // May as well consolidate it into one place.
+  let script = attrs => `<script ${attrs}><\/script>`;
+
+  let extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      "background": {"page": "background.html"},
+    },
+
+    files: {
+      "background.html": `
+        <meta charset="utf-8">
+        ${script('src="background.js" type="application/javascript"')}
+        ${script('src="background-1.js" type="application/javascript;version=1.8"')}
+        ${script('src="background-2.js" type="application/javascript;version=latest"')}
+        ${script('src="background-3.js" type="application/javascript"')}
+      `,
+
+      "background.js": function() {
+        window.reportResult = msg => {
+          browser.test.assertEq(
+            msg, "background-script-3",
+            "Expected a message only from the unversioned background script.");
+
+          browser.test.sendMessage("finished");
+        };
+      },
+
+      "background-1.js": function() {
+        window.reportResult("background-script-1");
+      },
+      "background-2.js": function() {
+        window.reportResult("background-script-2");
+      },
+      "background-3.js": function() {
+        window.reportResult("background-script-3");
+      },
+    },
+  });
+
+  consoleMonitor.start([{message: /Versioned JavaScript.*not supported in WebExtension.*developer\.mozilla\.org/},
+                        {message: /Versioned JavaScript.*not supported in WebExtension.*developer\.mozilla\.org/}]);
+
+  await Promise.all([extension.startup(),
+                     extension.awaitMessage("finished")]);
+
+  await consoleMonitor.finished();
+  await extension.unload();
+});
+
+</script>
+
+</body>
+</html>
diff --git a/toolkit/components/telemetry/Histograms.json b/toolkit/components/telemetry/Histograms.json
index 8d0255ae4161..7bbd0ebc6992 100644
--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -13862,6 +13862,14 @@
     "description": "Amount of time in milliseconds the main thread spends waiting for the paint thread to complete, if the time was greater than 200us.",
     "bug_numbers": [1386968]
   },
+  "SCRIPT_LOADED_WITH_VERSION": {
+    "record_in_processes": ["main", "content"],
+    "alert_emails": ["amarchesini@mozilla.com"],
+    "bug_numbers": [1418860],
+    "expires_in_version": "60",
+    "kind": "boolean",
+    "description": "Tracking how often scripts are loaded with a 'valid' version parameter. This telemetry ID helps us to decide if and when remove the support of script versioning."
+  },
   "STYLO_PARALLEL_RESTYLE_FRACTION": {
     "record_in_processes": ["content"],
     "alert_emails": ["manish@mozilla.com"],
diff --git a/uriloader/exthandler/tests/mochitest/download_page.html b/uriloader/exthandler/tests/mochitest/download_page.html
index 5837eeabda99..f02380f854c5 100644
--- a/uriloader/exthandler/tests/mochitest/download_page.html
+++ b/uriloader/exthandler/tests/mochitest/download_page.html
@@ -5,7 +5,7 @@
 <head>
   <meta charset=UTF-8>
   <title>Test page for link clicking</title>
-  <script type="text/javascript">
+  <script type="text/javascript; version=1.8">
     function launch_download() {
       window.open("download.bin", "_blank", "height=100,width=100");
     }
