Fill GWV with FCP from FF (#2020)

* Fill GWV with FCP from FF

* fix

Author: soulgalore

lib/firefox/webdriver/firefox.js

@@ -234,6 +234,15 @@ export class Firefox {
         result.browserScripts.timings.largestContentfulPaint.renderTime ||
         result.browserScripts.timings.largestContentfulPaint.loadTime;
     }
+
+    if (
+      result.browserScripts.timings &&
+      result.browserScripts.timings.paintTiming &&
+      result.browserScripts.timings.paintTiming['first-contentful-paint']
+    ) {
+      result.googleWebVitals.firstContentfulPaint =
+        result.browserScripts.timings.paintTiming['first-contentful-paint'];
+    }
   }
 
   async postWork(index, results) {