diff --git a/README.md b/README.md
index 2e64716..f53e5b3 100644
--- a/README.md
+++ b/README.md
@@ -1494,11 +1494,12 @@ wrote last, the record says which build's shape it is in.
 Each write reads the record, changes it and writes it back, holding a lock from
 the read to the write: the browser's own (`navigator.locks`), taken through
 `withLock()` in `src/shared/browserApi.js`. Every page of the extension and its
-background share it, except a page in a Firefox private window, which Firefox
-gives locks of its own (see https://git.eeqj.de/sneak/AutistMask/issues/646).
-The popup's `saveState()`, the background's `updateState()` and the
-StateRecovery reset all take it, so among the pages and the background that
-share it, no write lands between another write's read and its write, where it
+background share it, except a page in a Firefox private window, which has locks
+of its own. `manifest/firefox.json` sets `"incognito": "not_allowed"`, so such a
+page cannot read or write extension storage. In Chrome, a page opened from an
+incognito window still runs outside incognito and shares the lock. The popup's
+`saveState()`, the background's `updateState()` and the StateRecovery reset all
+take it, so no write lands between another write's read and its write, where it
 would be lost. Chrome has it from version 69, older than any Chrome that runs a
 Manifest V3 extension, in pages and in the service worker; Firefox from version
 96, older than the 115 `manifest/firefox.json` declares. Where it is missing, a
diff --git a/docs/README.md b/docs/README.md
index 8349567..4c0cccd 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -208,6 +208,9 @@ password.
 3. Click "Load Temporary Add-on".
 4. Select `dist/firefox/manifest.json`.
 
+AutistMask does not work in Firefox private windows. If Firefox is set to "Never
+remember history", every window is private, so AutistMask does not work at all.
+
 ## Getting Started
 
 ### Creating a New Wallet
diff --git a/manifest/firefox.json b/manifest/firefox.json
index 74b6e6f..08d8c6a 100644
--- a/manifest/firefox.json
+++ b/manifest/firefox.json
@@ -4,6 +4,7 @@
     "version": "0.1.0",
     "description": "Minimal Ethereum wallet for Firefox",
     "permissions": ["storage", "activeTab", "alarms", "<all_urls>"],
+    "incognito": "not_allowed",
     "content_security_policy": "default-src 'self'; script-src 'self' 'wasm-unsafe-eval'; object-src 'self'; style-src 'self'; img-src 'self' data:; connect-src 'self' https: http:; frame-src 'none'; form-action 'none'; base-uri 'none'",
     "icons": {
         "16": "icons/icon16.png",
diff --git a/src/background/state.js b/src/background/state.js
index 4f29804..3710d37 100644
--- a/src/background/state.js
+++ b/src/background/state.js
@@ -18,11 +18,10 @@
 //   updateState(fn)   read fresh, apply fn to that fresh record, write it
 //                     back — all holding the lock every write of the stored
 //                     record takes, so no other write lands in between, by
-//                     the background or by any page except a page in a
-//                     Firefox private window (README, "Stored state and its
-//                     version"). The read is one storage round trip ahead of
-//                     the write rather than a page lifetime ahead of it
-//                     (which is what made the popup's saveState() need a
+//                     the background or by any page (README, "Stored state
+//                     and its version"). The read is one storage round trip
+//                     ahead of the write rather than a page lifetime ahead of
+//                     it (which is what made the popup's saveState() need a
 //                     per-field merge against a baseline at all).
 //
 // A handler that must both read and write therefore does its network work
@@ -73,8 +72,7 @@ async function updateStateOnce(mutate) {
 // src/shared/browserApi.js) is held from the read to the write: two
 // background turns, or a turn and a page's save, never interleave
 // (https://git.eeqj.de/sneak/AutistMask/issues/641). Every page's save waits
-// while it is held, except a page in a Firefox private window, which Firefox
-// gives locks of its own (README, "Stored state and its version").
+// while it is held (README, "Stored state and its version").
 //
 // `mutate` must also not call updateState() itself, directly or through
 // anything it awaits: the inner turn waits for the lock the outer one holds,
diff --git a/src/popup/views/stateRecovery.js b/src/popup/views/stateRecovery.js
index 5449fed..0d56f5b 100644
--- a/src/popup/views/stateRecovery.js
+++ b/src/popup/views/stateRecovery.js
@@ -143,8 +143,6 @@ async function resetProfile() {
     try {
         // Under the lock every write of the stored record takes, so the erase
         // cannot land between another write's read and its write and be undone.
-        // A page in a Firefox private window is the exception: Firefox gives
-        // it locks of its own (README, "Stored state and its version").
         await withLock("autistmask", () => storageRemove("autistmask"));
     } catch (e) {
         log.errorf("state recovery: reset failed:", e);
diff --git a/src/shared/state.js b/src/shared/state.js
index 407c18f..227763c 100644
--- a/src/shared/state.js
+++ b/src/shared/state.js
@@ -581,9 +581,7 @@ function reportSaveFailure(err) {
 // Each save runs start to finish holding the lock every write of the stored
 // record takes (withLock() in src/shared/browserApi.js), so no write by
 // another page or the background lands between its read and its write and is
-// lost (https://git.eeqj.de/sneak/AutistMask/issues/641). A page in a Firefox
-// private window is the exception: Firefox gives it locks of its own (README,
-// "Stored state and its version").
+// lost (https://git.eeqj.de/sneak/AutistMask/issues/641).
 //
 // The same lock orders this page's own saves. showView() calls saveState() on
 // every navigation without awaiting it, so two saves from one page can be
diff --git a/tests/manifest.test.js b/tests/manifest.test.js
index af5cdf0..f00f55d 100644
--- a/tests/manifest.test.js
+++ b/tests/manifest.test.js
@@ -165,6 +165,15 @@ describe("declared icons", () => {
     });
 });
 
+// Firefox gives a page in a private window locks of its own, so that page's
+// save could land between the background's read and its write (README,
+// "Stored state and its version").
+describe("private windows", () => {
+    test("a page in a firefox private window cannot read or write extension storage", () => {
+        expect(readManifest("firefox").incognito).toBe("not_allowed");
+    });
+});
+
 describe("shipped Content Security Policy", () => {
     // MV3 takes an object and applies extension_pages to the popup and the
     // background service worker, which is where libsodium runs.
diff --git a/tests/stateWriteLock.test.js b/tests/stateWriteLock.test.js
index 041950c..3b37f9b 100644
--- a/tests/stateWriteLock.test.js
+++ b/tests/stateWriteLock.test.js
@@ -6,9 +6,8 @@
 // now holds a lock from its read to its write.
 //
 // The popup and the background run here as the real modules over one shared
-// storage stub, and share the lock the way the background and a page outside
-// a Firefox private window share the browser's
-// (tests/support/navigatorLocks.js).
+// storage stub, and share the lock the way the background and its pages share
+// the browser's (tests/support/navigatorLocks.js).
 
 const { makeStorageStub } = require("./support/storageStub");
 
diff --git a/tests/support/navigatorLocks.js b/tests/support/navigatorLocks.js
index f24f1b4..4722749 100644
--- a/tests/support/navigatorLocks.js
+++ b/tests/support/navigatorLocks.js
@@ -4,9 +4,8 @@
 // the callback settles, whether it resolved or rejected. That is what the
 // browser's own does for the one call the extension makes,
 // `navigator.locks.request(name, callback)`. Every module a test file loads
-// shares it, as the extension's background and its pages share the browser's,
-// except a page in a Firefox private window (README, "Stored state and its
-// version").
+// shares it, as the extension's background and its pages share the browser's
+// (README, "Stored state and its version").
 //
 // Run before each test file by `setupFiles` in package.json.