--- a/intl/l10n/L10nRegistry.jsm
+++ b/intl/l10n/L10nRegistry.jsm
@@ -320,16 +320,21 @@ class FileSource {
     //
     // If the `indexed` property is set to `true` it will be treated as the
     // resource not being available. Otherwise, the resource may be
     // available and we do not have any information about it yet.
     this.cache = {};
   }
 
   getPath(locale, path) {
+    // This is a special case for the only not BCP47-conformant locale
+    // code we have resources for.
+    if (locale === "ja-JP-macos") {
+      locale = "ja-JP-mac";
+    }
     return (this.prePath + path).replace(/\{locale\}/g, locale);
   }
 
   hasFile(locale, path) {
     if (!this.locales.includes(locale)) {
       return false;
     }
 

--- a/intl/l10n/Localization.jsm
+++ b/intl/l10n/Localization.jsm
@@ -96,17 +96,17 @@ class CachedIterable {
  * available in L10nRegistry, with locales requested by the user to
  * generate the iterator over MessageContexts.
  *
  * In the future, we may want to allow certain modules to override this
  * with a different negotitation strategy to allow for the module to
  * be localized into a different language - for example DevTools.
  */
 function defaultGenerateMessages(resourceIds) {
-  const appLocales = Services.locale.getAppLocalesAsLangTags();
+  const appLocales = Services.locale.getAppLocalesAsBCP47();
   return L10nRegistry.generateContexts(appLocales, resourceIds);
 }
 
 /**
  * The `Localization` class is a central high-level API for vanilla
  * JavaScript use of Fluent.
  * It combines language negotiation, MessageContext and I/O to
  * provide a scriptable API to format translations.

--- a/intl/locale/LocaleService.cpp
+++ b/intl/locale/LocaleService.cpp
@@ -39,27 +39,35 @@ NS_IMPL_ISUPPORTS(LocaleService, mozILoc
                   nsISupportsWeakReference)
 
 mozilla::StaticRefPtr<LocaleService> LocaleService::sInstance;
 
 /**
  * This function transforms a canonical Mozilla Language Tag, into it's
  * BCP47 compilant form.
  *
- * Example: "ja-JP-mac" -> "ja-JP-x-lvariant-mac"
+ * Example: "ja-JP-mac" -> "ja-JP-macos"
  *
  * The BCP47 form should be used for all calls to ICU/Intl APIs.
  * The canonical form is used for all internal operations.
  */
 static bool
 SanitizeForBCP47(nsACString& aLocale, bool strict)
 {
   // Currently, the only locale code we use that's not BCP47-conformant is
-  // "ja-JP-mac" on OS X, but let's try to be more general than just
-  // hard-coding that here.
+  // "ja-JP-mac" on OS X, and ICU canonicalizes it into a mouthfull
+  // "ja-JP-x-lvariant-mac", so instead we're hardcoding a conversion
+  // of it to "ja-JP-macos".
+  if (aLocale.LowerCaseEqualsASCII("ja-jp-mac")) {
+    aLocale.AssignLiteral("ja-JP-macos");
+    return true;
+  }
+
+  // The rest of this function will use ICU canonicalization for any other
+  // tag that may come this way.
   const int32_t LANG_TAG_CAPACITY = 128;
   char langTag[LANG_TAG_CAPACITY];
   nsAutoCString locale(aLocale);
   locale.Trim(" ");
   UErrorCode err = U_ZERO_ERROR;
   // This is a fail-safe method that will set langTag to "und" if it cannot
   // match any part of the input locale code.
   int32_t len = uloc_toLanguageTag(locale.get(), langTag, LANG_TAG_CAPACITY,