diff --git a/browser/actors/LightweightThemeChild.jsm b/browser/actors/LightweightThemeChild.jsm
new file mode 100644
index 000000000000..3fd3675664ff
--- /dev/null
+++ b/browser/actors/LightweightThemeChild.jsm
@@ -0,0 +1,72 @@
+/* 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/. */
+
+"use strict";
+
+var EXPORTED_SYMBOLS = ["LightweightThemeChild"];
+
+ChromeUtils.import("resource://gre/modules/ActorChild.jsm");
+ChromeUtils.import("resource://gre/modules/Services.jsm");
+
+/**
+ * LightweightThemeChild forwards theme data to in-content pages.
+ */
+class LightweightThemeChild extends ActorChild {
+  constructor(mm) {
+    super(mm);
+
+    this.init();
+  }
+
+  /**
+   * Initializes the actor for the current page, sending it any existing
+   * theme data, and adding shared data change listeners so it can
+   * notify the page of future updates.
+   *
+   * This is called when the actor is constructed, and any time
+   * ActorManagerChild receives a pageshow event for the page we're
+   * attached to.
+   */
+  init() {
+    Services.cpmm.sharedData.addEventListener("change", this);
+    this.update(this.mm.chromeOuterWindowID, this.content);
+  }
+
+  /**
+   * Cleans up any global listeners registered by the actor.
+   *
+   * This is called by ActorManagerChild any time it receives a pagehide
+   * event for the page we're attached to.
+   */
+  cleanup() {
+    Services.cpmm.sharedData.removeEventListener("change", this);
+  }
+
+  /**
+   * Handles "change" events on the child sharedData map, and notifies
+   * our content page if its theme data was among the changed keys.
+   */
+  handleEvent(event) {
+    if (event.type === "change") {
+      if (event.changedKeys.includes(`theme/${this.mm.chromeOuterWindowID}`)) {
+        this.update(this.mm.chromeOuterWindowID, this.content);
+      }
+    }
+  }
+
+  /**
+   * Forward the theme data to the page.
+   * @param {Object} outerWindowID The outerWindowID the parent process window has.
+   * @param {Object} content The receiving global
+   */
+  update(outerWindowID, content) {
+    const event = Cu.cloneInto({
+      detail: {
+        data: Services.cpmm.sharedData.get(`theme/${outerWindowID}`)
+      },
+    }, content);
+    content.dispatchEvent(new content.CustomEvent("LightweightTheme:Set",
+                                                  event));
+  }
+}
diff --git a/browser/actors/moz.build b/browser/actors/moz.build
index 4e241dee27e6..a9a1175915f0 100644
--- a/browser/actors/moz.build
+++ b/browser/actors/moz.build
@@ -7,6 +7,9 @@
 with Files("**"):
     BUG_COMPONENT = ("Firefox", "General")
 
+with Files("LightweightThemeChild.jsm"):
+    BUG_COMPONENT = ("WebExtensions", "Themes")
+
 with Files("LightWeightThemeInstallChild.jsm"):
     BUG_COMPONENT = ("Firefox", "Theme")
 
@@ -30,6 +33,7 @@ FINAL_TARGET_FILES.actors += [
     'ContentSearchChild.jsm',
     'ContextMenuChild.jsm',
     'DOMFullscreenChild.jsm',
+    'LightweightThemeChild.jsm',
     'LightWeightThemeInstallChild.jsm',
     'LinkHandlerChild.jsm',
     'NetErrorChild.jsm',
diff --git a/browser/base/content/tab-content.js b/browser/base/content/tab-content.js
index 7fdb8dbbb72e..9f67d9689462 100644
--- a/browser/base/content/tab-content.js
+++ b/browser/base/content/tab-content.js
@@ -22,22 +22,6 @@ ActorManagerChild.attach(this, "browsers");
 // TabChildGlobal
 var global = this;
 
-XPCOMUtils.defineLazyProxy(this, "LightweightThemeChildHelper",
-  "resource:///modules/LightweightThemeChildHelper.jsm");
-
-let themeablePagesWhitelist = new Set([
-  "about:home",
-  "about:newtab",
-  "about:welcome",
-]);
-
-addEventListener("pageshow", function({ originalTarget }) {
-  if (originalTarget.defaultView == content && themeablePagesWhitelist.has(content.document.documentURI)) {
-    LightweightThemeChildHelper.listen(themeablePagesWhitelist);
-    LightweightThemeChildHelper.update(chromeOuterWindowID, content);
-  }
-}, false, true);
-
 // Keep a reference to the translation content handler to avoid it it being GC'ed.
 var trHandler = null;
 if (Services.prefs.getBoolPref("browser.translation.detectLanguage")) {
diff --git a/browser/components/nsBrowserGlue.js b/browser/components/nsBrowserGlue.js
index 557096ea8783..9b8b914b6cb7 100644
--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -131,6 +131,16 @@ let ACTORS = {
     },
   },
 
+  LightweightTheme: {
+    child: {
+      module: "resource:///actors/LightweightThemeChild.jsm",
+      matches: ["about:home", "about:newtab", "about:welcome"],
+      events: {
+        "pageshow": {mozSystemGroup: true},
+      },
+    },
+  },
+
   LinkHandler: {
     child: {
       module: "resource:///actors/LinkHandlerChild.jsm",
diff --git a/browser/modules/LightweightThemeChildHelper.jsm b/browser/modules/LightweightThemeChildHelper.jsm
deleted file mode 100644
index 35ca2eeb3880..000000000000
--- a/browser/modules/LightweightThemeChildHelper.jsm
+++ /dev/null
@@ -1,68 +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/. */
-
-"use strict";
-
-ChromeUtils.import("resource://gre/modules/Services.jsm");
-
-var EXPORTED_SYMBOLS = ["LightweightThemeChildHelper"];
-
-/**
- * LightweightThemeChildHelper forwards theme data to in-content pages.
- */
-var LightweightThemeChildHelper = {
-  listener: null,
-  whitelist: null,
-
-  /**
-   * Listen to theme updates for the current process
-   * @param {Array} whitelist The pages that can receive theme updates.
-   */
-  listen(whitelist) {
-    if (!this.listener) {
-      // Clone the whitelist to avoid leaking the global the whitelist
-      // originates from.
-      this.whitelist = new Set([...whitelist]);
-      this.listener = ({ changedKeys }) => {
-        if (changedKeys.find(change => change.startsWith("theme/"))) {
-          this._updateProcess(changedKeys);
-        }
-      };
-      Services.cpmm.sharedData.addEventListener("change", this.listener);
-    }
-  },
-
-  /**
-   * Update the theme data for the whole process
-   * @param {Array} changedKeys The sharedData keys that were changed.
-   */
-  _updateProcess(changedKeys) {
-    const windowEnumerator = Services.ww.getWindowEnumerator();
-    while (windowEnumerator.hasMoreElements()) {
-      const {
-        chromeOuterWindowID,
-        content,
-      } = windowEnumerator.getNext().docShell.messageManager;
-      if (changedKeys.includes(`theme/${chromeOuterWindowID}`) &&
-          content && this.whitelist.has(content.document.documentURI)) {
-        this.update(chromeOuterWindowID, content);
-      }
-    }
-  },
-
-  /**
-   * Forward the theme data to the page.
-   * @param {Object} outerWindowID The outerWindowID the parent process window has.
-   * @param {Object} content The receiving global
-   */
-  update(outerWindowID, content) {
-    const event = Cu.cloneInto({
-      detail: {
-        data: Services.cpmm.sharedData.get(`theme/${outerWindowID}`)
-      },
-    }, content);
-    content.dispatchEvent(new content.CustomEvent("LightweightTheme:Set",
-                                                  event));
-  },
-};
diff --git a/browser/modules/moz.build b/browser/modules/moz.build
index 40f1ef7900df..72c3dc6fc938 100644
--- a/browser/modules/moz.build
+++ b/browser/modules/moz.build
@@ -61,9 +61,6 @@ with Files("ExtensionsUI.jsm"):
 with Files("LaterRun.jsm"):
     BUG_COMPONENT = ("Firefox", "Tours")
 
-with Files("LightweightThemeChildHelper.jsm"):
-    BUG_COMPONENT = ("WebExtensions", "Themes")
-
 with Files("OpenInTabsUtils.jsm"):
     BUG_COMPONENT = ("Firefox", "Tabbed Browser")
 
@@ -134,7 +131,6 @@ EXTRA_JS_MODULES += [
     'FormValidationHandler.jsm',
     'HomePage.jsm',
     'LaterRun.jsm',
-    'LightweightThemeChildHelper.jsm',
     'OpenInTabsUtils.jsm',
     'PageActions.jsm',
     'PermissionUI.jsm',
diff --git a/toolkit/modules/ActorManagerChild.jsm b/toolkit/modules/ActorManagerChild.jsm
index e424d4ff9f26..4c8596a21d32 100644
--- a/toolkit/modules/ActorManagerChild.jsm
+++ b/toolkit/modules/ActorManagerChild.jsm
@@ -116,13 +116,27 @@ class SingletonDispatcher extends Dispatcher {
   constructor(window, data) {
     super(getMessageManager(window), data);
 
-    window.addEventListener("pageshow", this);
-    window.addEventListener("pagehide", this);
+    window.addEventListener("pageshow", this, {mozSystemGroup: true});
+    window.addEventListener("pagehide", this, {mozSystemGroup: true});
 
     this.window = window;
     this.listeners = [];
   }
 
+  init() {
+    super.init();
+
+    for (let actor of this.instances.values()) {
+      if (typeof actor.init === "function") {
+        try {
+          actor.init();
+        } catch (e) {
+          Cu.reportError(e);
+        }
+      }
+    }
+  }
+
   cleanup() {
     super.cleanup();
 
@@ -132,6 +146,17 @@ class SingletonDispatcher extends Dispatcher {
     for (let [event, listener, options] of this.listeners) {
       this.window.removeEventListener(event, listener, options);
     }
+
+    for (let actor of this.instances.values()) {
+      if (typeof actor.cleanup === "function") {
+        try {
+          actor.cleanup();
+        } catch (e) {
+          Cu.reportError(e);
+        }
+      }
+    }
+
     this.listeners = null;
   }