This happens to me, about 20% of the time. Just re-running Firefox works for me, not always on 1st retry, but before too long.
If that doesn't work for you, try: Open Avast!>RealtimeShields>ProcessVirtualization>ExpertSettings. Check the box that says "All Settings (inc. extensions, add-ons, etc.)"
This is just a work-around, makes the sandbox not quite as secure, but it lets my flash and other video elements load 100% of the time.
Fix for this is supposed to come with ver. 5.1