fix: reduce Firefox's concurrency to 1 to avoid unreliable focus (#35)

Author: dlockhart

src/server/wtr-config.js

@@ -213,6 +213,7 @@ export class WTRConfig {
 		if (!Array.isArray(browsers)) throw new TypeError('browsers must be an array');
 
 		return browsers.map((b) => playwrightLauncher({
+			concurrency: b === 'firefox' ? 1 : undefined, // focus in Firefox unreliable if concurrency > 1 (https://github.com/modernweb-dev/web/issues/238)
 			product: b,
 			createBrowserContext: ({ browser }) => browser.newContext({ deviceScaleFactor: 2, reducedMotion: 'reduce' })
 		}));