--- a/intl/l10n/L10nRegistry.jsm
+++ b/intl/l10n/L10nRegistry.jsm
@@ -1,8 +1,9 @@
+const { AppConstants } = ChromeUtils.import("resource://gre/modules/AppConstants.jsm", {});
 const { Services } = ChromeUtils.import('resource://gre/modules/Services.jsm', {});
 const { MessageContext } = ChromeUtils.import("resource://gre/modules/MessageContext.jsm", {});
 Components.utils.importGlobalProperties(["fetch"]); /* globals fetch */
 
 /**
  * L10nRegistry is a localization resource management system for Gecko.
  *
  * It manages the list of resource sources provided with the app and allows
@@ -213,16 +214,38 @@ async function* generateContextsForLocal
     } else {
       // otherwise recursively load another generator that walks over the
       // partially resolved list of sources.
       yield * generateContextsForLocale(locale, sourcesOrder, resourceIds, order);
     }
   }
 }
 
+const  MSG_CONTEXT_OPTIONS = {
+  functions: {
+    /**
+     * PLATFORM is a built-in allowing localizers to differentiate message
+     * variants depending on the target platform.
+     */
+    PLATFORM: () => {
+      switch (AppConstants.platform) {
+        case "linux":
+        case "android":
+          return AppConstants.platform;
+        case "win":
+          return "windows";
+        case "macosx":
+          return "macos";
+        default:
+          return "other";
+      }
+    }
+  }
+}
+
 /**
  * Generates a single MessageContext by loading all resources
  * from the listed sources for a given locale.
  *
  * The function casts all error cases into a Promise that resolves with
  * value `null`.
  * This allows the caller to be an async generator without using
  * try/catch clauses.
@@ -239,17 +262,17 @@ function generateContext(locale, sources
   }
 
   const fetchPromises = resourceIds.map((resourceId, i) => {
     return L10nRegistry.sources.get(sourcesOrder[i]).fetchFile(locale, resourceId);
   });
 
   const ctxPromise = Promise.all(fetchPromises).then(
     dataSets => {
-      const ctx = new MessageContext(locale);
+      const ctx = new MessageContext(locale, MSG_CONTEXT_OPTIONS);
       for (const data of dataSets) {
         if (data === null) {
           return null;
         }
         ctx.addMessages(data);
       }
       return ctx;
     },

--- a/intl/l10n/test/test_localization.js
+++ b/intl/l10n/test/test_localization.js
@@ -1,11 +1,12 @@
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
+const { AppConstants } = ChromeUtils.import("resource://gre/modules/AppConstants.jsm", {});
 const { Localization } = ChromeUtils.import("resource://gre/modules/Localization.jsm", {});
 
 add_task(function test_methods_presence() {
   equal(typeof Localization.prototype.formatValues, "function");
   equal(typeof Localization.prototype.formatMessages, "function");
   equal(typeof Localization.prototype.formatValue, "function");
 });
 
@@ -25,17 +26,17 @@ add_task(async function test_methods_cal
 
   L10nRegistry.load = async function(url) {
     return fs[url];
   }
 
   const source = new FileSource('test', ['de', 'en-US'], '/localization/{locale}');
   L10nRegistry.registerSource(source);
 
-  async function * generateMessages(resIds) {
+  async function* generateMessages(resIds) {
     yield * await L10nRegistry.generateContexts(['de', 'en-US'], resIds);
   }
 
   const l10n = new Localization([
     '/browser/menu.ftl'
   ], generateMessages);
 
   let values = await l10n.formatValues([['key'], ['key2']]);
@@ -43,8 +44,52 @@ add_task(async function test_methods_cal
   equal(values[0], '[de] Value2');
   equal(values[1], '[en] Value3');
 
   L10nRegistry.sources.clear();
   L10nRegistry.load = originalLoad;
   LocaleService.setRequestedLocales(originalRequested);
 });
 
+add_task(async function test_builtins() {
+  const { L10nRegistry, FileSource } =
+    Components.utils.import("resource://gre/modules/L10nRegistry.jsm", {});
+
+  const known_platforms = {
+    'linux': 'linux',
+    'win': 'windows',
+    'macosx': 'macos',
+    'android': 'android',
+  };
+
+  const fs = {
+    '/localization/en-US/test.ftl': `
+key = { PLATFORM() ->
+        ${ Object.values(known_platforms).map(
+              name => `      [${ name }] ${ name.toUpperCase() } Value\n`).join('') }
+       *[other] OTHER Value
+    }`,
+  };
+  const originalLoad = L10nRegistry.load;
+
+  L10nRegistry.load = async function(url) {
+    return fs[url];
+  }
+
+  const source = new FileSource('test', ['en-US'], '/localization/{locale}');
+  L10nRegistry.registerSource(source);
+
+  async function* generateMessages(resIds) {
+    yield * await L10nRegistry.generateContexts(['en-US'], resIds);
+  }
+
+  const l10n = new Localization([
+    '/test.ftl'
+  ], generateMessages);
+
+  let values = await l10n.formatValues([['key']]);
+
+  ok(values[0].includes(
+    `${ known_platforms[AppConstants.platform].toUpperCase() } Value`));
+
+  L10nRegistry.sources.clear();
+  L10nRegistry.load = originalLoad;
+});