fix(firefox): force fast shutdown after "xpcom-will-shutdown" phase (#18556)

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

@@ -93,6 +93,10 @@ export class Firefox extends BrowserType {
 // Prefs for quick fixes that didn't make it to the build.
 // Should all be moved to `playwright.cfg`.
 const kBandaidFirefoxUserPrefs = {
+  // Avoid stalling on shutdown, after "xpcom-will-shutdown" phase.
+  // This at least happens when shutting down soon after launching.
+  // See AppShutdown.cpp for more details on shutdown phases.
+  'toolkit.shutdown.fastShutdownStage': 3,
 };
 
 const kDisableFissionFirefoxUserPrefs = {