diff --git a/content/base/src/nsContentUtils.cpp b/content/base/src/nsContentUtils.cpp
index b8246cd03531..8e2d8b1d87f7 100644
--- a/content/base/src/nsContentUtils.cpp
+++ b/content/base/src/nsContentUtils.cpp
@@ -3591,7 +3591,7 @@ nsContentUtils::CreateContextualFragment(nsIDOMNode* aContextNode,
   nsCOMPtr<nsIHTMLDocument> htmlDoc(do_QueryInterface(document));
   PRBool bHTML = htmlDoc && !bCaseSensitive;
 
-  if (bHTML && nsContentUtils::GetBoolPref("html5.enable", PR_TRUE)) {
+  if (bHTML && nsContentUtils::GetBoolPref("html5.enable", PR_FALSE)) {
     // See if the document has a cached fragment parser. nsHTMLDocument is the
     // only one that should really have one at the moment.
     nsCOMPtr<nsIParser> parser = document->GetFragmentParser();
diff --git a/content/html/document/src/nsHTMLDocument.cpp b/content/html/document/src/nsHTMLDocument.cpp
index be3735980164..53113199a333 100644
--- a/content/html/document/src/nsHTMLDocument.cpp
+++ b/content/html/document/src/nsHTMLDocument.cpp
@@ -656,7 +656,7 @@ nsHTMLDocument::StartDocumentLoad(const char* aCommand,
                                   PRBool aReset,
                                   nsIContentSink* aSink)
 {
-  PRBool loadAsHtml5 = nsContentUtils::GetBoolPref("html5.enable", PR_TRUE);
+  PRBool loadAsHtml5 = nsContentUtils::GetBoolPref("html5.enable", PR_FALSE);
   if (aSink) {
     loadAsHtml5 = PR_FALSE;
   }
@@ -1806,7 +1806,7 @@ nsHTMLDocument::OpenCommon(const nsACString& aContentType, PRBool aReplace)
     return NS_ERROR_DOM_NOT_SUPPORTED_ERR;
   }
 
-  PRBool loadAsHtml5 = nsContentUtils::GetBoolPref("html5.enable", PR_TRUE);
+  PRBool loadAsHtml5 = nsContentUtils::GetBoolPref("html5.enable", PR_FALSE);
 
   nsresult rv = NS_OK;
 
diff --git a/modules/libpref/src/init/all.js b/modules/libpref/src/init/all.js
index 8132abeaa77b..9c005bd9d087 100644
--- a/modules/libpref/src/init/all.js
+++ b/modules/libpref/src/init/all.js
@@ -2733,3 +2733,6 @@ pref("mozilla.widget.disable-native-theme", true);
 
 // Enable/Disable the geolocation API for content
 pref("geo.enabled", true);
+
+// Enable/Disable HTML5 parser
+pref("html5.enable", false);