diff --git a/toolkit/components/extensions/moz.build b/toolkit/components/extensions/moz.build
index 614b05507adf..76afd553ce6c 100644
--- a/toolkit/components/extensions/moz.build
+++ b/toolkit/components/extensions/moz.build
@@ -15,3 +15,5 @@ EXTRA_JS_MODULES += [
 JAR_MANIFESTS += ['jar.mn']
 
 MOCHITEST_MANIFESTS += ['test/mochitest/mochitest.ini']
+
+XPCSHELL_TESTS_MANIFESTS += ['test/xpcshell/xpcshell.ini']
diff --git a/toolkit/components/extensions/test/xpcshell/head.js b/toolkit/components/extensions/test/xpcshell/head.js
new file mode 100644
index 000000000000..71823e9e8284
--- /dev/null
+++ b/toolkit/components/extensions/test/xpcshell/head.js
@@ -0,0 +1,10 @@
+"use strict";
+
+const { classes: Cc, interfaces: Ci, utils: Cu, results: Cr } = Components;
+
+Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "NetUtil",
+                                  "resource://gre/modules/NetUtil.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Services",
+                                  "resource://gre/modules/Services.jsm");
diff --git a/toolkit/components/extensions/test/xpcshell/test_locale_converter.js b/toolkit/components/extensions/test/xpcshell/test_locale_converter.js
new file mode 100644
index 000000000000..4b3f6c4e156a
--- /dev/null
+++ b/toolkit/components/extensions/test/xpcshell/test_locale_converter.js
@@ -0,0 +1,118 @@
+"use strict";
+
+const convService = Cc["@mozilla.org/streamConverters;1"]
+  .getService(Ci.nsIStreamConverterService);
+
+const UUID = "72b61ee3-aceb-476c-be1b-0822b036c9f1";
+const ADDON_ID = "test@web.extension";
+const URI = NetUtil.newURI(`moz-extension://${UUID}/file.css`);
+
+const FROM_TYPE = "application/vnd.mozilla.webext.unlocalized";
+const TO_TYPE = "text/css";
+
+
+function StringStream(string) {
+  let stream = Cc["@mozilla.org/io/string-input-stream;1"]
+    .createInstance(Ci.nsIStringInputStream);
+
+  stream.data = string;
+  return stream;
+}
+
+
+// Initialize the policy service with a stub localizer for our
+// add-on ID.
+add_task(function* init() {
+  const aps = Cc["@mozilla.org/addons/policy-service;1"]
+    .getService(Ci.nsIAddonPolicyService).wrappedJSObject;
+
+  let oldCallback = aps.setExtensionURIToAddonIdCallback(uri => {
+    if (uri.host == UUID) {
+      return ADDON_ID;
+    }
+  });
+
+  aps.setAddonLocalizeCallback(ADDON_ID, string => {
+    return string.replace(/__MSG_(.*?)__/g, "<localized-$1>");
+  });
+
+  do_register_cleanup(() => {
+    aps.setExtensionURIToAddonIdCallback(oldCallback);
+    aps.setAddonLocalizeCallback(ADDON_ID, null);
+  });
+});
+
+
+// Test that the synchronous converter works as expected with a
+// simple string.
+add_task(function* testSynchronousConvert() {
+  let stream = StringStream("Foo __MSG_xxx__ bar __MSG_yyy__ baz");
+
+  let resultStream = convService.convert(stream, FROM_TYPE, TO_TYPE, URI);
+
+  let result = NetUtil.readInputStreamToString(resultStream, resultStream.available());
+
+  equal(result, "Foo <localized-xxx> bar <localized-yyy> baz");
+});
+
+
+// Test that the asynchronous converter works as expected with input
+// split into multiple chunks, and a boundary in the middle of a
+// replacement token.
+add_task(function* testAsyncConvert() {
+  let listener;
+  let awaitResult = new Promise((resolve, reject) => {
+    listener = {
+      QueryInterface: XPCOMUtils.generateQI([Ci.nsIStreamListener]),
+
+      onDataAvailable(request, context, inputStream, offset, count) {
+        this.resultParts.push(NetUtil.readInputStreamToString(inputStream, count));
+      },
+
+      onStartRequest() {
+        ok(!("resultParts" in this));
+        this.resultParts = [];
+      },
+
+      onStopRequest(request, context, statusCode) {
+        if (!Components.isSuccessCode(statusCode)) {
+          reject(new Error(statusCode));
+        }
+
+        resolve(this.resultParts.join("\n"));
+      },
+    };
+  });
+
+  let parts = ["Foo __MSG_x", "xx__ bar __MSG_yyy__ baz"];
+
+  let converter = convService.asyncConvertData(FROM_TYPE, TO_TYPE, listener, URI);
+  converter.onStartRequest(null, null);
+
+  for (let part of parts) {
+    converter.onDataAvailable(null, null, StringStream(part), 0, part.length);
+  }
+
+  converter.onStopRequest(null, null, Cr.NS_OK);
+
+
+  let result = yield awaitResult;
+  equal(result, "Foo <localized-xxx> bar <localized-yyy> baz");
+});
+
+
+// Test that attempting to initialize a converter with the URI of a
+// nonexistent WebExtension fails.
+add_task(function* testInvalidUUID() {
+  let uri = NetUtil.newURI("moz-extension://eb4f3be8-41c9-4970-aa6d-b84d1ecc02b2/file.css");
+
+  Assert.throws(() => {
+    convService.convert(stream, FROM_TYPE, TO_TYPE, uri);
+  });
+
+  Assert.throws(() => {
+    let listener = { QueryInterface: XPCOMUtils.generateQI([Ci.nsIStreamListener]) };
+
+    convService.asyncConvertData(FROM_TYPE, TO_TYPE, listener, uri);
+  });
+});
diff --git a/toolkit/components/extensions/test/xpcshell/xpcshell.ini b/toolkit/components/extensions/test/xpcshell/xpcshell.ini
new file mode 100644
index 000000000000..ea2f7d8a63d7
--- /dev/null
+++ b/toolkit/components/extensions/test/xpcshell/xpcshell.ini
@@ -0,0 +1,5 @@
+[DEFAULT]
+head = head.js
+tail =
+
+[test_locale_converter.js]