--- a/browser/components/preferences/in-content/privacy.js
+++ b/browser/components/preferences/in-content/privacy.js
@@ -835,20 +835,24 @@ var gPrivacyPane = {
   },
 
   // PRIVATE BROWSING
 
   /**
    * Initialize the starting state for the auto-start private browsing mode pref reverter.
    */
   initAutoStartPrivateBrowsingReverter() {
+    // We determine the mode in initializeHistoryMode, which is guaranteed to have been
+    // called before now, so this is up-to-date.
     let mode = document.getElementById("historyMode");
-    let autoStart = document.getElementById("privateBrowsingAutoStart");
     this._lastMode = mode.selectedIndex;
-    this._lastCheckState = autoStart.hasAttribute("checked");
+    // The value of the autostart pref, on the other hand, is gotten from Preferences,
+    // which updates the DOM asynchronously, so we can't rely on the DOM. Get it directly
+    // from the prefs.
+    this._lastCheckState = Preferences.get("browser.privatebrowsing.autostart").value;
   },
 
   _lastMode: null,
   _lastCheckState: null,
   async updateAutostart() {
     let mode = document.getElementById("historyMode");
     let autoStart = document.getElementById("privateBrowsingAutoStart");
     let pref = Preferences.get("browser.privatebrowsing.autostart");

--- a/browser/components/preferences/in-content/tests/browser.ini
+++ b/browser/components/preferences/in-content/tests/browser.ini
@@ -86,12 +86,13 @@ skip-if = e10s
 support-files =
   subdialog.xul
   subdialog2.xul
 [browser_sync_sanitize.js]
 skip-if = os == 'win' && processor == "x86_64" && bits == 64 # bug 1522821
 [browser_telemetry.js]
 # Skip this test on Android as FHR and Telemetry are separate systems there.
 skip-if = !healthreport || !telemetry || (os == 'linux' && debug) || (os == 'android')
+[browser_warning_permanent_private_browsing.js]
 [browser_containers_name_input.js]
 run-if = nightly_build # Containers is enabled only on Nightly
 [browser_fluent.js]
 [browser_hometab_restore_defaults.js]

new file mode 100644
--- /dev/null
+++ b/browser/components/preferences/in-content/tests/browser_warning_permanent_private_browsing.js
@@ -0,0 +1,45 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+function checkForPrompt(prefVal) {
+  return async function() {
+    await SpecialPowers.pushPrefEnv({set: [
+      ["privacy.history.custom", true],
+      ["browser.privatebrowsing.autostart", !prefVal],
+    ]});
+
+    await openPreferencesViaOpenPreferencesAPI("panePrivacy", { leaveOpen: true });
+    let doc = gBrowser.contentDocument;
+    is(doc.getElementById("historyMode").value, "custom", "Expect custom history mode");
+
+    // Stub out the prompt method as an easy way to check it was shown. We throw away
+    // the tab straight after so don't need to bother restoring it.
+    let promptFired = false;
+    doc.defaultView.confirmRestartPrompt = () => {
+      promptFired = true;
+      return doc.defaultView.CONFIRM_RESTART_PROMPT_RESTART_NOW;
+    };
+    // Tick the checkbox and pretend the user did it:
+    let checkbox = doc.getElementById("privateBrowsingAutoStart");
+    checkbox.checked = prefVal;
+    checkbox.doCommand();
+
+    // Now the prompt should have shown.
+    ok(promptFired,
+      `Expect a prompt when turning permanent private browsing ${prefVal ? "on" : "off"}!`);
+    BrowserTestUtils.removeTab(gBrowser.selectedTab);
+  };
+}
+
+/**
+ * Check we show the prompt if the permanent private browsing pref is false
+ * and we flip the checkbox to true.
+ */
+add_task(checkForPrompt(true));
+
+/**
+ * Check it works in the other direction:
+ */
+add_task(checkForPrompt(false));