Skip flaky FF test that was set to pending

spec/selenium_spec_firefox_remote.rb

@@ -73,7 +73,8 @@ Capybara::SpecHelper.run_specs TestSessions::RemoteFirefox, FIREFOX_REMOTE_DRIVE
   when 'Capybara::Session selenium_firefox_remote #reset_session! removes ALL cookies'
     pending "Geckodriver doesn't provide a way to remove cookies outside the current domain"
   when /#accept_confirm should work with nested modals$/
-    pending 'Broken in FF 63 - https://bugzilla.mozilla.org/show_bug.cgi?id=1487358' if marionette_gte?(63, @session)
+    # skip because this is timing based and hence flaky when set to pending
+    skip 'Broken in FF 63 - https://bugzilla.mozilla.org/show_bug.cgi?id=1487358' if marionette_gte?(63, @session)
   end
 end