diff --git a/browser/base/content/moz.build b/browser/base/content/moz.build
index 8efe140ef0f7..8e2914502952 100644
--- a/browser/base/content/moz.build
+++ b/browser/base/content/moz.build
@@ -79,6 +79,9 @@ with Files("test/siteIdentity/**"):
 with Files("test/sidebar/**"):
     BUG_COMPONENT = ("Firefox", "General")
 
+with Files("test/startup/**"):
+    BUG_COMPONENT = ("Firefox", "General")
+
 with Files("test/static/**"):
     BUG_COMPONENT = ("Firefox", "General")
 
diff --git a/browser/base/content/test/startup/.eslintrc.js b/browser/base/content/test/startup/.eslintrc.js
new file mode 100644
index 000000000000..1779fd7f1cf8
--- /dev/null
+++ b/browser/base/content/test/startup/.eslintrc.js
@@ -0,0 +1,5 @@
+"use strict";
+
+module.exports = {
+  extends: ["plugin:mozilla/browser-test"],
+};
diff --git a/browser/base/content/test/startup/browser.ini b/browser/base/content/test/startup/browser.ini
new file mode 100644
index 000000000000..0924965c2e9d
--- /dev/null
+++ b/browser/base/content/test/startup/browser.ini
@@ -0,0 +1,2 @@
+[browser_preXULSkeletonUIRegistry.js]
+skip-if = os != 'win'
\ No newline at end of file
diff --git a/browser/base/content/test/startup/browser_preXULSkeletonUIRegistry.js b/browser/base/content/test/startup/browser_preXULSkeletonUIRegistry.js
new file mode 100644
index 000000000000..5d3e8d36f912
--- /dev/null
+++ b/browser/base/content/test/startup/browser_preXULSkeletonUIRegistry.js
@@ -0,0 +1,107 @@
+ChromeUtils.defineModuleGetter(
+  this,
+  "WindowsRegistry",
+  "resource://gre/modules/WindowsRegistry.jsm"
+);
+
+// This is copied from WindowsRegistry.jsm, but extended to support
+// TYPE_BINARY, as that is how we represent doubles in the registry for
+// the skeleton UI. However, we didn't extend WindowsRegistry.jsm itself,
+// because TYPE_BINARY is kind of a footgun for javascript callers - our
+// use case is just trivial (checking that the value is non-zero).
+function readRegKeyExtended(aRoot, aPath, aKey, aRegistryNode = 0) {
+  const kRegMultiSz = 7;
+  const kMode = Ci.nsIWindowsRegKey.ACCESS_READ | aRegistryNode;
+  let registry = Cc["@mozilla.org/windows-registry-key;1"].createInstance(
+    Ci.nsIWindowsRegKey
+  );
+  try {
+    registry.open(aRoot, aPath, kMode);
+    if (registry.hasValue(aKey)) {
+      let type = registry.getValueType(aKey);
+      switch (type) {
+        case kRegMultiSz:
+          // nsIWindowsRegKey doesn't support REG_MULTI_SZ type out of the box.
+          let str = registry.readStringValue(aKey);
+          return str.split("\0").filter(v => v);
+        case Ci.nsIWindowsRegKey.TYPE_STRING:
+          return registry.readStringValue(aKey);
+        case Ci.nsIWindowsRegKey.TYPE_INT:
+          return registry.readIntValue(aKey);
+        case Ci.nsIWindowsRegKey.TYPE_BINARY:
+          return registry.readBinaryValue(aKey);
+        default:
+          throw new Error("Unsupported registry value.");
+      }
+    }
+  } catch (ex) {
+  } finally {
+    registry.close();
+  }
+  return undefined;
+}
+
+add_task(async function testWritesEnabledOnPrefChange() {
+  Services.prefs.setBoolPref("browser.startup.preXulSkeletonUI", true);
+  let enabled = WindowsRegistry.readRegKey(
+    Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+    "Software\\Mozilla\\Firefox\\PreXULSkeletonUISettings",
+    "enabled"
+  );
+  is(enabled, 1, "Pre-XUL skeleton UI is enabled in the Windows registry");
+
+  Services.prefs.setBoolPref("browser.startup.preXulSkeletonUI", false);
+  enabled = WindowsRegistry.readRegKey(
+    Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+    "Software\\Mozilla\\Firefox\\PreXULSkeletonUISettings",
+    "enabled"
+  );
+  is(enabled, 0, "Pre-XUL skeleton UI is disabled in the Windows registry");
+});
+
+add_task(async function testWritesSizeValuesOnChange() {
+  // Enable the skeleton UI, since if it's disabled we won't persist the size values
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.startup.preXulSkeletonUI", true]],
+  });
+
+  const regKeys = [
+    "width",
+    "height",
+    "screenX",
+    "screenY",
+    "cssToDevPixelScaling",
+  ];
+
+  // Remove all of the registry values to ensure old tests aren't giving us false
+  // positives
+  for (let key of regKeys) {
+    WindowsRegistry.removeRegKey(
+      Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+      "Software\\Mozilla\\Firefox\\PreXULSkeletonUISettings",
+      key
+    );
+  }
+
+  const win = await BrowserTestUtils.openNewBrowserWindow();
+  for (let key of regKeys) {
+    let value = readRegKeyExtended(
+      Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+      "Software\\Mozilla\\Firefox\\PreXULSkeletonUISettings",
+      key
+    );
+    ok(
+      value,
+      `Skeleton UI registry values should have a non-zero value for ${key}`
+    );
+    if (value.length) {
+      let hasNonZero = false;
+      for (var i = 0; i < value.length; i++) {
+        hasNonZero = hasNonZero || value[i];
+      }
+      ok(hasNonZero, `Value should have non-zero components for ${key}`);
+    }
+  }
+
+  await BrowserTestUtils.closeWindow(win);
+});
diff --git a/browser/base/moz.build b/browser/base/moz.build
index fdfa93d4623a..7d12024f2516 100644
--- a/browser/base/moz.build
+++ b/browser/base/moz.build
@@ -49,6 +49,7 @@ BROWSER_CHROME_MANIFESTS += [
     'content/test/sanitize/browser.ini',
     'content/test/sidebar/browser.ini',
     'content/test/siteIdentity/browser.ini',
+    'content/test/startup/browser.ini',
     'content/test/static/browser.ini',
     'content/test/statuspanel/browser.ini',
     'content/test/sync/browser.ini',