diff --git a/js/packages/truapi/src/sandbox.test.ts b/js/packages/truapi/src/sandbox.test.ts
index 523e66bc..288dfd13 100644
--- a/js/packages/truapi/src/sandbox.test.ts
+++ b/js/packages/truapi/src/sandbox.test.ts
@@ -135,6 +135,29 @@ describe("sandbox iframe MessagePort handshake", () => {
     expect(currentWindow.listeners.size).toBe(0);
   });
 
+  it('treats a masked "null" ancestor origin as hidden and pings with the wildcard', async () => {
+    // Firefox implements location.ancestorOrigins but serializes cross-origin
+    // ancestors as "null", which is not a valid postMessage targetOrigin.
+    currentWindow = installFakeIframeWindow({ ancestorOrigins: ["null"] });
+    const sandbox = await importSandbox();
+
+    expect(sandbox.getClientSync()).not.toBeNull();
+    expect(currentWindow.parentPostMessage.mock.calls).toEqual([
+      [{ type: "truapi-ready" }, "*"],
+    ]);
+
+    const accepted = trackChannel();
+    currentWindow.dispatch({
+      source: currentWindow.parent,
+      origin: "https://host.example",
+      data: { type: "truapi-init" },
+      ports: [accepted.port1],
+    });
+    await Promise.resolve();
+    expect(currentWindow.win.__HOST_API_PORT__).toBe(accepted.port1);
+    expect(currentWindow.listeners.size).toBe(0);
+  });
+
   it("uses a data-free wildcard ready ping only when the host origin is hidden", async () => {
     currentWindow = installFakeIframeWindow({});
     const sandbox = await importSandbox();
diff --git a/js/packages/truapi/src/sandbox.ts b/js/packages/truapi/src/sandbox.ts
index 99ebdef4..33a8b189 100644
--- a/js/packages/truapi/src/sandbox.ts
+++ b/js/packages/truapi/src/sandbox.ts
@@ -68,8 +68,10 @@ function resolveHostOrigin(): string | null {
       // Fall through to ancestorOrigins.
     }
   }
-  const ancestors = window.location?.ancestorOrigins;
-  if (ancestors && ancestors.length > 0) return ancestors[0] ?? null;
+  // Firefox serializes cross-origin ancestors as "null", which is not a
+  // valid postMessage targetOrigin; treat it as an unknown host origin.
+  const ancestor = window.location?.ancestorOrigins?.[0];
+  if (ancestor && ancestor !== "null") return ancestor;
   return null;
 }