Bug 1319024 - Remove usage of browser.newtab.url from Firefox Puppeteer. r=maja_zf

MozReview-Commit-ID: LwMJgQMgJoP

--HG--
extra : rebase_source : 81e0883a19df46c710c6546cde0fb469464e664e

testing/puppeteer/firefox/firefox_puppeteer/mixins.py

@@ -83,9 +83,11 @@ class PuppeteerMixin(object):
         self.puppeteer = Puppeteer(self.marionette)
         self.browser = self.puppeteer.windows.current
         self.browser.focus()
+
         with self.marionette.using_context(self.marionette.CONTEXT_CONTENT):
-            # Ensure that we have a default page opened
-            self.marionette.navigate(self.puppeteer.prefs.get_pref('browser.newtab.url'))
+            # Bug 1312674 - Navigating to about:blank twice can cause a hang in
+            # Marionette. So try to always have a known default page loaded.
+            self.marionette.navigate('about:')
 
     def tearDown(self, *args, **kwargs):
         self.marionette.set_context('chrome')