mchain
64
Well, could be either the default settings within Avast! Online Security or user modified (enable all) settings are interfering.
Ad Tracking when enabled is known to cause video streaming issues on certain specific sites that require adverts to run video streaming properly. Disabling it for only those sites that require adverts be allowed fixes the vid streaming issue on that site. Maybe something similar for other tracking categories for Firefox and those specific sites? Try disabling one category at a time and see what happens.
When you leave that site, you can re-enable the Ad Tracking feature.