In the past week or so, Youtube has made changes that broke the playing of 3D videos in Internet Explorer and Firefox. Videos appear to display properly in Chrome and Opera browser.
In IE and Firefox, 3D videos display like this, rather than in the full size of the viewer:
Al Caudullo (3DGuy.tv) has found a work around: Go to each of your 3D videos on Youtube, and edit the “Tags” section and add:
I did this for all of my 3D videos and this did fix the problem.