chore: housekeeping after the last firefox roll (#35116)

packages/playwright-core/src/server/firefox/ffBrowser.ts

@@ -444,5 +444,4 @@ function toJugglerProxyOptions(proxy: types.ProxySettings) {
 // Prefs for quick fixes that didn't make it to the build.
 // Should all be moved to `playwright.cfg`.
 const kBandaidFirefoxUserPrefs = {
-  'dom.fetchKeepalive.enabled': false,
 };

tests/library/browsercontext-basic.spec.ts

@@ -342,3 +342,10 @@ it('default user agent', async ({ launchPersistent, browser, page, mode }) => {
   const { userAgent } = await (browser as any)._channel.defaultUserAgentForTest();
   expect(await page.evaluate(() => navigator.userAgent)).toBe(userAgent);
 });
+
+it('should create two pages in parallel', async ({ context }) => {
+  await Promise.all([
+    context.newPage(),
+    context.newPage(),
+  ]);
+});