--- a/dom/encoding/FallbackEncoding.cpp
+++ b/dom/encoding/FallbackEncoding.cpp
@@ -50,16 +50,17 @@ static const EncodingProp domainsFallbac
 static constexpr nsUConvProp nonParticipatingDomains[] = {
 #include "nonparticipatingdomains.properties.h"
 };
 
 NS_IMPL_ISUPPORTS(FallbackEncoding, nsIObserver)
 
 FallbackEncoding* FallbackEncoding::sInstance = nullptr;
 bool FallbackEncoding::sGuessFallbackFromTopLevelDomain = true;
+bool FallbackEncoding::sFallbackToUTF8ForFile = false;
 
 FallbackEncoding::FallbackEncoding()
   : mFallback(nullptr)
 {
   MOZ_ASSERT(!FallbackEncoding::sInstance,
              "Singleton already exists.");
 }
 
@@ -150,16 +151,18 @@ FallbackEncoding::Initialize()
   MOZ_ASSERT(!FallbackEncoding::sInstance,
              "Initializing pre-existing fallback cache.");
   FallbackEncoding::sInstance = new FallbackEncoding;
   Preferences::RegisterCallback(FallbackEncoding::PrefChanged,
                                 "intl.charset.fallback.override",
                                 nullptr);
   Preferences::AddBoolVarCache(&sGuessFallbackFromTopLevelDomain,
                                "intl.charset.fallback.tld");
+  Preferences::AddBoolVarCache(&sFallbackToUTF8ForFile,
+                               "intl.charset.fallback.utf8_for_file");
 
   nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
   if (obs) {
     obs->AddObserver(sInstance, "intl:requested-locales-changed", true);
   }
 }
 
 void

--- a/dom/encoding/FallbackEncoding.h
+++ b/dom/encoding/FallbackEncoding.h
@@ -22,16 +22,21 @@ public:
   NS_DECL_NSIOBSERVER
 
   /**
    * Whether FromTopLevelDomain() should be used.
    */
   static bool sGuessFallbackFromTopLevelDomain;
 
   /**
+   * Whether UTF-8 should be used for file URLs.
+   */
+  static bool sFallbackToUTF8ForFile;
+
+  /**
    * Gets the locale-dependent fallback encoding for legacy HTML and plain
    * text content.
    *
    * @param aFallback the outparam for the fallback encoding
    */
   static NotNull<const Encoding*> FromLocale();
 
   /**

--- a/dom/html/nsHTMLDocument.cpp
+++ b/dom/html/nsHTMLDocument.cpp
@@ -481,16 +481,22 @@ nsHTMLDocument::TryTLD(int32_t& aCharset
 void
 nsHTMLDocument::TryFallback(int32_t& aCharsetSource,
                             NotNull<const Encoding*>& aEncoding)
 {
   if (kCharsetFromFallback <= aCharsetSource)
     return;
 
   aCharsetSource = kCharsetFromFallback;
+  bool isFile = false;
+  if (FallbackEncoding::sFallbackToUTF8ForFile && mDocumentURI &&
+      NS_SUCCEEDED(mDocumentURI->SchemeIs("file", &isFile)) && isFile) {
+    aEncoding = UTF_8_ENCODING;
+    return;
+  }
   aEncoding = FallbackEncoding::FromLocale();
 }
 
 void
 nsHTMLDocument::SetDocumentCharacterSet(NotNull<const Encoding*> aEncoding)
 {
   nsDocument::SetDocumentCharacterSet(aEncoding);
   // Make sure to stash this charset on our channel as needed if it's a wyciwyg

--- a/dom/html/nsHTMLDocument.h
+++ b/dom/html/nsHTMLDocument.h
@@ -357,18 +357,18 @@ protected:
                             NotNull<const Encoding*>& aEncoding);
   static void TryCacheCharset(nsICachingChannel* aCachingChannel,
                               int32_t& aCharsetSource,
                               NotNull<const Encoding*>& aEncoding);
   void TryParentCharset(nsIDocShell*  aDocShell,
                         int32_t& charsetSource,
                         NotNull<const Encoding*>& aEncoding);
   void TryTLD(int32_t& aCharsetSource, NotNull<const Encoding*>& aCharset);
-  static void TryFallback(int32_t& aCharsetSource,
-                          NotNull<const Encoding*>& aEncoding);
+  void TryFallback(int32_t& aCharsetSource,
+                   NotNull<const Encoding*>& aEncoding);
 
   // Override so we can munge the charset on our wyciwyg channel as needed.
   virtual void
     SetDocumentCharacterSet(NotNull<const Encoding*> aEncoding) override;
 
   // Tracks if we are currently processing any document.write calls (either
   // implicit or explicit). Note that if a write call writes out something which
   // would block the parser, then mWriteLevel will be incorrect until the parser

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -2282,16 +2282,17 @@ pref("converter.html2txt.header_strategy
 pref("converter.html2txt.always_include_ruby", false);
 
 pref("intl.accept_languages",               "chrome://global/locale/intl.properties");
 pref("intl.menuitems.alwaysappendaccesskeys","chrome://global/locale/intl.properties");
 pref("intl.menuitems.insertseparatorbeforeaccesskeys","chrome://global/locale/intl.properties");
 pref("intl.charset.detector",               "chrome://global/locale/intl.properties");
 pref("intl.charset.fallback.override",      "");
 pref("intl.charset.fallback.tld",           true);
+pref("intl.charset.fallback.utf8_for_file", false);
 pref("intl.ellipsis",                       "chrome://global-platform/locale/intl.properties");
 pref("intl.locale.matchOS",                 false);
 // this pref allows user to request that all internationalization formatters
 // like date/time formatting, unit formatting, calendars etc. should use
 // OS locale set instead of the app locale set.
 pref("intl.regional_prefs.use_os_locales",  false);
 // fallback charset list for Unicode conversion (converting from Unicode)
 // currently used for mail send only to handle symbol characters (e.g Euro, trademark, smartquotes)