diff --git a/packages/playwright-core/src/server/firefox/firefox.ts b/packages/playwright-core/src/server/firefox/firefox.ts
index a55d28750bb0e..a4062fa375d27 100644
--- a/packages/playwright-core/src/server/firefox/firefox.ts
+++ b/packages/playwright-core/src/server/firefox/firefox.ts
@@ -68,7 +68,7 @@ export class Firefox extends BrowserType {
       throw new Error('Pass userDataDir parameter to `browserType.launchPersistentContext(userDataDir, ...)` instead of specifying --profile argument');
     if (args.find(arg => arg.startsWith('-juggler')))
       throw new Error('Use the port parameter instead of -juggler argument');
-    const firefoxUserPrefs = isPersistent ? undefined : options.firefoxUserPrefs;
+    const firefoxUserPrefs = isPersistent ? undefined : { ...kBandaidFirefoxUserPrefs, ...options.firefoxUserPrefs };
     if (firefoxUserPrefs) {
       const lines: string[] = [];
       for (const [name, value] of Object.entries(firefoxUserPrefs))
@@ -92,3 +92,9 @@ export class Firefox extends BrowserType {
     return firefoxArguments;
   }
 }
+
+// Prefs for quick fixes that didn't make it to the build.
+// Should all be moved to `playwright.cfg`.
+const kBandaidFirefoxUserPrefs = {
+  'network.cookie.cookieBehavior': 4,
+};
diff --git a/tests/library/browsercontext-cookies.spec.ts b/tests/library/browsercontext-cookies.spec.ts
index e1d139a76846a..342be6b059875 100644
--- a/tests/library/browsercontext-cookies.spec.ts
+++ b/tests/library/browsercontext-cookies.spec.ts
@@ -320,3 +320,32 @@ it('should add cookies with an expiration', async ({ context }) => {
     expires: -42,
   }])).rejects.toThrow(/Cookie should have a valid expires/);
 });
+
+it('should be able to send third party cookies via an iframe', async ({ browser, httpsServer, browserName, isMac }) => {
+  it.fixme(browserName === 'webkit' && isMac);
+  it.info().annotations.push({ type: 'issue', description: 'https://github.com/microsoft/playwright/issues/16937' });
+
+  const context = await browser.newContext({
+    ignoreHTTPSErrors: true,
+  });
+  try {
+    const page = await context.newPage();
+    await page.goto(httpsServer.EMPTY_PAGE);
+    await context.addCookies([{
+      domain: new URL(httpsServer.CROSS_PROCESS_PREFIX).hostname,
+      path: '/',
+      name: 'cookie1',
+      value: 'yes',
+      httpOnly: true,
+      secure: true,
+      sameSite: 'None'
+    }]);
+    const [response] = await Promise.all([
+      httpsServer.waitForRequest('/grid.html'),
+      page.setContent(`<iframe src="${httpsServer.CROSS_PROCESS_PREFIX}/grid.html"></iframe>`)
+    ]);
+    expect(response.headers['cookie']).toBe('cookie1=yes');
+  } finally {
+    await context.close();
+  }
+});