--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -3672,16 +3672,17 @@ nsIDocument::DefaultStyleAttrURLData()
   return mCachedURLData;
 }
 
 void
 nsIDocument::SetDocumentCharacterSet(NotNull<const Encoding*> aEncoding)
 {
   if (mCharacterSet != aEncoding) {
     mCharacterSet = aEncoding;
+    mEncodingMenuDisabled = aEncoding == UTF_8_ENCODING;
 
     if (nsPresContext* context = GetPresContext()) {
       context->DispatchCharSetChange(aEncoding);
     }
   }
 }
 
 void

--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -1108,19 +1108,19 @@ public:
   {
     mSandboxFlags = sandboxFlags;
   }
 
   /**
    * Called when the document was decoded as UTF-8 and decoder encountered no
    * errors.
    */
-  void DisableEncodingMenu()
+  void EnableEncodingMenu()
   {
-    mEncodingMenuDisabled = true;
+    mEncodingMenuDisabled = false;
   }
 
   /**
    * Access HTTP header data (this may also get set from other
    * sources, like HTML META tags).
    */
   void GetHeaderData(nsAtom* aHeaderField, nsAString& aData) const;
   void SetHeaderData(nsAtom* aheaderField, const nsAString& aData);

--- a/parser/html/nsHtml5StreamParser.cpp
+++ b/parser/html/nsHtml5StreamParser.cpp
@@ -830,17 +830,22 @@ nsHtml5StreamParser::WriteStreamBytes(co
   for (;;) {
     auto dst = mLastBuffer->TailAsSpan(NS_HTML5_STREAM_PARSER_READ_BUFFER_SIZE);
     uint32_t result;
     size_t read;
     size_t written;
     bool hadErrors;
     Tie(result, read, written, hadErrors) =
       mUnicodeDecoder->DecodeToUTF16(src, dst, false);
-    mHasHadErrors |= hadErrors;
+    if (hadErrors && !mHasHadErrors) {
+      mHasHadErrors = true;
+      if (mEncoding == UTF_8_ENCODING) {
+        mTreeBuilder->TryToEnableEncodingMenu();
+      }
+    }
     src = src.From(read);
     totalRead += read;
     mLastBuffer->AdvanceEnd(written);
     if (result == kOutputFull) {
       RefPtr<nsHtml5OwningUTF16Buffer> newBuf =
         nsHtml5OwningUTF16Buffer::FalliblyCreate(
           NS_HTML5_STREAM_PARSER_READ_BUFFER_SIZE);
       if (!newBuf) {
@@ -1081,17 +1086,22 @@ nsHtml5StreamParser::DoStopRequest()
   for (;;) {
     auto dst = mLastBuffer->TailAsSpan(NS_HTML5_STREAM_PARSER_READ_BUFFER_SIZE);
     uint32_t result;
     size_t read;
     size_t written;
     bool hadErrors;
     Tie(result, read, written, hadErrors) =
       mUnicodeDecoder->DecodeToUTF16(src, dst, true);
-    mHasHadErrors |= hadErrors;
+    if (hadErrors && !mHasHadErrors) {
+      mHasHadErrors = true;
+      if (mEncoding == UTF_8_ENCODING) {
+        mTreeBuilder->TryToEnableEncodingMenu();
+      }
+    }
     MOZ_ASSERT(read == 0, "How come an empty span was read form?");
     mLastBuffer->AdvanceEnd(written);
     if (result == kOutputFull) {
       RefPtr<nsHtml5OwningUTF16Buffer> newBuf =
         nsHtml5OwningUTF16Buffer::FalliblyCreate(
           NS_HTML5_STREAM_PARSER_READ_BUFFER_SIZE);
       if (!newBuf) {
         MarkAsBroken(NS_ERROR_OUT_OF_MEMORY);
@@ -1437,19 +1447,16 @@ nsHtml5StreamParser::ParseAvailableData(
               }
             }
             return; // no more data for now but expecting more
           case STREAM_ENDED:
             if (mAtEOF) {
               return;
             }
             mAtEOF = true;
-            if (mEncoding == UTF_8_ENCODING && !mHasHadErrors) {
-              mTreeBuilder->TryToDisableEncodingMenu();
-            }
             if (mCharsetSource < kCharsetFromMetaTag) {
               if (mInitialEncodingWasFromParentFrame) {
                 // Unfortunately, this check doesn't take effect for
                 // cross-origin frames, so cross-origin ad frames that have
                 // no text and only an image or a Flash embed get the more
                 // severe message from the next if block. The message is
                 // technically accurate, though.
                 mTreeBuilder->MaybeComplainAboutCharset(

--- a/parser/html/nsHtml5TreeBuilderCppSupplement.h
+++ b/parser/html/nsHtml5TreeBuilderCppSupplement.h
@@ -1303,25 +1303,25 @@ nsHtml5TreeBuilder::MaybeComplainAboutCh
   if (MOZ_UNLIKELY(mBuilder)) {
     MOZ_ASSERT_UNREACHABLE("Must never complain about charset with builder.");
     return;
   }
   mOpQueue.AppendElement()->Init(aMsgId, aError, aLineNumber);
 }
 
 void
-nsHtml5TreeBuilder::TryToDisableEncodingMenu()
+nsHtml5TreeBuilder::TryToEnableEncodingMenu()
 {
   if (MOZ_UNLIKELY(mBuilder)) {
     MOZ_ASSERT_UNREACHABLE("Must never disable encoding menu with builder.");
     return;
   }
   nsHtml5TreeOperation* treeOp = mOpQueue.AppendElement();
   NS_ASSERTION(treeOp, "Tree op allocation failed.");
-  treeOp->Init(eTreeOpDisableEncodingMenu);
+  treeOp->Init(eTreeOpEnableEncodingMenu);
 }
 
 void
 nsHtml5TreeBuilder::AddSnapshotToScript(nsAHtml5TreeBuilderState* aSnapshot,
                                         int32_t aLine)
 {
   if (MOZ_UNLIKELY(mBuilder)) {
     MOZ_ASSERT_UNREACHABLE("Must never use snapshots with builder.");

--- a/parser/html/nsHtml5TreeBuilderHSupplement.h
+++ b/parser/html/nsHtml5TreeBuilderHSupplement.h
@@ -139,17 +139,17 @@ void
 NeedsCharsetSwitchTo(NotNull<const Encoding*> aEncoding,
                      int32_t aSource,
                      int32_t aLineNumber);
 
 void
 MaybeComplainAboutCharset(const char* aMsgId, bool aError, int32_t aLineNumber);
 
 void
-TryToDisableEncodingMenu();
+TryToEnableEncodingMenu();
 
 void
 AddSnapshotToScript(nsAHtml5TreeBuilderState* aSnapshot, int32_t aLine);
 
 void
 DropHandles();
 
 void

--- a/parser/html/nsHtml5TreeOperation.cpp
+++ b/parser/html/nsHtml5TreeOperation.cpp
@@ -1052,19 +1052,19 @@ nsHtml5TreeOperation::Perform(nsHtml5Tre
     }
     case eTreeOpMaybeComplainAboutCharset: {
       char* msgId = mOne.charPtr;
       bool error = mTwo.integer;
       int32_t lineNumber = mThree.integer;
       aBuilder->MaybeComplainAboutCharset(msgId, error, (uint32_t)lineNumber);
       return NS_OK;
     }
-    case eTreeOpDisableEncodingMenu: {
+    case eTreeOpEnableEncodingMenu: {
       nsIDocument* doc = aBuilder->GetDocument();
-      doc->DisableEncodingMenu();
+      doc->EnableEncodingMenu();
       return NS_OK;
     }
     case eTreeOpAddClass: {
       Element* element = (*(mOne.node))->AsElement();
       char16_t* str = mTwo.unicharPtr;
       nsDependentString depStr(str);
       // See viewsource.css for the possible classes
       nsAutoString klass;

--- a/parser/html/nsHtml5TreeOperation.h
+++ b/parser/html/nsHtml5TreeOperation.h
@@ -64,17 +64,17 @@ enum eHtml5TreeOperation
   eTreeOpSvgLoad,
   eTreeOpMaybeComplainAboutCharset,
   eTreeOpAddClass,
   eTreeOpAddViewSourceHref,
   eTreeOpAddViewSourceBase,
   eTreeOpAddError,
   eTreeOpAddLineNumberId,
   eTreeOpStartLayout,
-  eTreeOpDisableEncodingMenu
+  eTreeOpEnableEncodingMenu
 };
 
 class nsHtml5TreeOperationStringPair
 {
 private:
   nsString mPublicId;
   nsString mSystemId;