I'm running Firefox myself and I've had something similar on and off for the last couple of days. I thought maybe it was something to do with my having to use my Mum's laptop while the pc is in drydock for repair, or perhaps Youtube being a bit glitchy.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'