Disabling Hardware graphics acceleration did the trick !

I lose the smooth scrolling but the videos seem to be working fine so far. I guess my video card isn't that well supported on Firefox.

Why do Youtube videos start black?
In the Peter Jackson version of The Return of the King, Merry offers Théoden his sword and service, which Théoden accepts. Théoden gives Merry the title of "Meriadoc -- Esquire of Rohan." 

When the Rohirrim set out to ride to Minas Tirith, Théoden is very firm with Merry, that he is not allowed to ride into battle, and tells Merry that he is a burden that none of the Rohirrim can bear at that time. 

Why didn't Théoden let Merry ride to war? And yes, I know that Merry was taken into the Battle of the Pellenor Fields by Éowyn, who herself was going to fight on the sly. 
