--- a/toolkit/components/extensions/ExtensionCommon.jsm
+++ b/toolkit/components/extensions/ExtensionCommon.jsm
@@ -1495,19 +1495,17 @@ LocaleData.prototype = {
 
   get acceptLanguages() {
     let result = Preferences.get("intl.accept_languages", "", Ci.nsIPrefLocalizedString);
     return result.split(/\s*,\s*/g);
   },
 
 
   get uiLocale() {
-    // Return the browser locale, but convert it to a Chrome-style
-    // locale code.
-    return Services.locale.getAppLocaleAsBCP47().replace(/-/g, "_");
+    return Services.locale.getAppLocaleAsBCP47();
   },
 };
 
 defineLazyGetter(LocaleData.prototype, "availableLocales", function() {
   return new Set([this.BUILTIN, this.selectedLocale, this.defaultLocale]
                  .filter(locale => this.messages.has(locale)));
 });
 

--- a/toolkit/components/extensions/test/xpcshell/test_ext_i18n.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_i18n.js
@@ -365,17 +365,17 @@ add_task(async function test_get_ui_lang
     },
   });
 
   let contentPage = await ExtensionTestUtils.loadContentPage(`${BASE_URL}/file_sample.html`);
 
   await extension.startup();
   await extension.awaitMessage("content-loaded");
 
-  extension.sendMessage(["expect-results", "en_US"]);
+  extension.sendMessage(["expect-results", "en-US"]);
 
   await extension.awaitMessage("background-done");
   await extension.awaitMessage("content-done");
 
   // We don't currently have a good way to mock this.
   if (false) {
     Preferences.set("general.useragent.locale", "he");
 

--- a/toolkit/components/extensions/test/xpcshell/test_ext_i18n_css.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_i18n_css.js
@@ -110,17 +110,17 @@ async function test_i18n_css(options = {
 
   equal(maxWidth, "42px", "stylesheet correctly applied");
 
   await contentPage.close();
 
   cssURL = cssURL.replace(/foo.css$/, "locale.css");
 
   css = await fetch(cssURL);
-  equal(css, '* { content: "en_US ltr rtl left right" }', "CSS file localized in mochitest scope");
+  equal(css, '* { content: "en-US ltr rtl left right" }', "CSS file localized in mochitest scope");
 
   // We don't currently have a good way to mock this.
   if (false) {
     const LOCALE = "general.useragent.locale";
     const DIR = "intl.uidirection";
     const DIR_LEGACY = "intl.uidirection.en"; // Needed for Android until bug 1215247 is resolved
 
     // We don't wind up actually switching the chrome registry locale, since we