Ok more on this.
It was not a Firefox add-on because the behaviour continued even after disabling all Firefox add-ons.
The problem was this:
I had two different plugins competing for facebook opengraph attention at the same time.
- Social login
http://www.oneall.com/- Jetpack par WordPress.com
To solve the problem, I disabled only the component of Jet Pack that was trying to connect to Facebook opengraph to get login information. The first two on the upper left. Notifications and Annoucements. They are not necessary with Social Login installed.
Now everything is fine.
And the issue was NOT with Fest theme.