--- a/devtools/startup/aboutdevtools/subscribe.js
+++ b/devtools/startup/aboutdevtools/subscribe.js
@@ -5,42 +5,37 @@
 "use strict";
 
 /**
  * This file handles the newsletter subscription form on about:devtools.
  * It is largely inspired from https://mozilla.github.io/basket-example/
  */
 
 window.addEventListener("load", function() {
-  const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm", {});
-
   // Timeout for the subscribe XHR.
   const REQUEST_TIMEOUT = 5000;
 
-  const ABOUTDEVTOOLS_STRINGS = "chrome://devtools-startup/locale/aboutdevtools.properties";
-  const aboutDevtoolsBundle = Services.strings.createBundle(ABOUTDEVTOOLS_STRINGS);
-
   const emailInput = document.getElementById("email");
   const newsletterErrors = document.getElementById("newsletter-errors");
   const newsletterForm = document.getElementById("newsletter-form");
   const newsletterPrivacySection = document.getElementById("newsletter-privacy");
   const newsletterThanks = document.getElementById("newsletter-thanks");
 
   /**
    * Update the error panel to display the provided errors. If the argument is null or
    * empty, a default error message will be displayed.
    *
    * @param {Array} errors
    *        Array of strings, each item being an error message to display.
    */
-  function updateErrorPanel(errors) {
+  async function updateErrorPanel(errors) {
     clearErrorPanel();
 
     if (!errors || errors.length == 0) {
-      errors = [aboutDevtoolsBundle.GetStringFromName("newsletter.error.unknown")];
+      errors = [await document.l10n.formatValues([{id: "newsletter-error-unknown"}])];
     }
 
     // Create errors markup.
     const fragment = document.createDocumentFragment();
     for (const error of errors) {
       const item = document.createElement("p");
       item.classList.add("error");
       item.appendChild(document.createTextNode(error));
@@ -87,43 +82,46 @@ window.addEventListener("load", function
     evt.preventDefault();
     evt.stopPropagation();
 
     // New submission, clear old errors
     clearErrorPanel();
 
     const xhr = new XMLHttpRequest();
 
-    xhr.onload = function(r) {
+    xhr.onload = async function(r) {
       if (r.target.status >= 200 && r.target.status < 300) {
         const {response} = r.target;
 
         if (response.success === true) {
           // Hide form and show success message.
           newsletterForm.style.display = "none";
           newsletterThanks.classList.add("show");
         } else {
           // We trust the error messages from the service to be meaningful for the user.
           updateErrorPanel(response.errors);
         }
       } else {
         const {status, statusText} = r.target;
         const statusInfo = `${status} - ${statusText}`;
-        const error = aboutDevtoolsBundle
-          .formatStringFromName("newsletter.error.common", [statusInfo], 1);
+        const error = await document.l10n.formatValues([
+          { id: "newsletter-error-common", args: { errorDescription: statusInfo } },
+        ]);
         updateErrorPanel([error]);
       }
     };
 
     xhr.onerror = () => {
       updateErrorPanel();
     };
 
-    xhr.ontimeout = () => {
-      const error = aboutDevtoolsBundle.GetStringFromName("newsletter.error.timeout");
+    xhr.ontimeout = async () => {
+      const error = await document.l10n.formatValues([
+        { id: "newsletter-error-timeout" },
+      ]);
       updateErrorPanel([error]);
     };
 
     const url = newsletterForm.getAttribute("action");
 
     xhr.open("POST", url, true);
     xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
     xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");

--- a/devtools/startup/locales/en-US/aboutDevTools.ftl
+++ b/devtools/startup/locales/en-US/aboutDevTools.ftl
@@ -43,10 +43,15 @@ features-responsive-title = Responsive D
 features-responsive-desc = Test sites on emulated devices in your browser. <a data-l10n-name="learn-more">{ features-learn-more }</a>
 features-visual-editing-title = Visual Editing
 features-visual-editing-desc = Fine-tune animations, alignment and padding. <a data-l10n-name="learn-more">{ features-learn-more }</a>
 features-performance-title = Performance
 features-performance-desc = Unblock bottlenecks, streamline processes, optimize assets. <a data-l10n-name="learn-more">{ features-learn-more }</a>
 features-memory-title = Memory
 features-memory-desc = Find memory leaks and make your application zippy. <a data-l10n-name="learn-more">{ features-learn-more }</a>
 # Variables:
+#   $errorDescription (String) - The error that occurred e.g. 404 - Not Found
+newsletter-error-common = Subscription request failed ({ $errorDescription }).
+newsletter-error-unknown = An unexpected error occurred.
+newsletter-error-timeout = Subscription request timed out.
+# Variables:
 #   $shortcut (String) - The keyboard shortcut used for the tool
 welcome-message = You’ve successfully enabled Developer Tools! To get started, explore the Web Developer menu or open the tools with { $shortcut }.

deleted file mode 100644
--- a/devtools/startup/locales/en-US/aboutdevtools.properties
+++ /dev/null
@@ -1,18 +0,0 @@
-# This Source Code Form is subject to the terms of the Mozilla Public
-# License, v. 2.0. If a copy of the MPL was not distributed with this
-# file, You can obtain one at http://mozilla.org/MPL/2.0/.
-
-# LOCALIZATION NOTE These strings are used in the about:devtools page.
-
-# LOCALIZATION NOTE (newsletter.error.common): error text displayed when the newsletter
-# subscription failed. The argument will be replaced with request's status code and status
-# text (e.g. "404 - Not Found")
-newsletter.error.common=Subscription request failed (%S).
-
-# LOCALIZATION NOTE (newsletter.error.unknown): error text displayed when the newsletter
-# subscription failed for an unexpected reason.
-newsletter.error.unknown=An unexpected error occurred.
-
-# LOCALIZATION NOTE (newsletter.error.timeout): error text displayed when the newsletter
-# subscription timed out.
-newsletter.error.timeout=Subscription request timed out.

new file mode 100644
--- /dev/null
+++ b/python/l10n/fluent_migrations/bug_1504457_aboutdevtools.py
@@ -0,0 +1,40 @@
+from __future__ import absolute_import
+import fluent.syntax.ast as FTL
+from fluent.migrate.helpers import transforms_from
+from fluent.migrate.helpers import VARIABLE_REFERENCE
+from fluent.migrate.helpers import TERM_REFERENCE
+from fluent.migrate.helpers import MESSAGE_REFERENCE
+from fluent.migrate import REPLACE
+from fluent.migrate import COPY
+from fluent.migrate import CONCAT
+
+def migrate(ctx):
+    """Bug 1504457 -  Migrate subscribe.js strings of about:devtools to Fluent, part {index}"""
+
+    ctx.add_transforms(
+        "devtools/startup/aboutDevTools.ftl",
+        "devtools/startup/aboutDevTools.ftl",
+        transforms_from(
+"""
+newsletter-error-unknown = { COPY("devtools/startup/aboutdevtools.properties", "newsletter.error.unknown")}
+newsletter-error-timeout = { COPY("devtools/startup/aboutdevtools.properties", "newsletter.error.timeout")}
+"""
+        )
+    )
+
+    ctx.add_transforms(
+        "devtools/startup/aboutDevTools.ftl",
+        "devtools/startup/aboutDevTools.ftl",
+        [
+            FTL.Message(
+                id=FTL.Identifier("newsletter-error-common"),
+                value=REPLACE(
+                    "devtools/startup/aboutdevtools.properties",
+                    "newsletter.error.common",
+                    {
+                        "%S": VARIABLE_REFERENCE("errorDescription")
+                    }
+                )
+            )
+        ]
+    )