Spoke too soon! OK now this is crazy. It is working sometimes. I finally have symptoms that change when i try different things, but those symptoms are changing.
FIRST: If I accessed youtube as "youtube.com" then the videos worked, but if I accessed them as "www.youtube.com" they are blank.
Clicking on this link worked 15 minutes ago but no longer does: http://youtube.com/watch?v=n_O0KGqicSo
but this has always failed (everything on the page appears except for the actual video and play button) http://www.youtube.com/watch?v=n_O0KGqicSo
I clicked on the link to the youtube video (no www name) you posted, and it (as expected) opened a new tab with the video page, and sure enough the video was there and it played. If I surf youtube in that tab, everything worked. If I selected a video and "open in new tab" or "open in new window" it worked. But if I opened my own new tab (CTRL-T) and paste a URL for a youtube video, it would only work if I omitted the "www". but now suddenly that ain't working any more.
Then I thought maybe I have to open a new tab or window from a "previously working " tab or window. and that held true for about 10 minutes. Now its all failing again.
EITHER URL works in netscape. Has not failed once. Same for IE. so there is definitely something really weird going on with my firefox. the only thing I can think of trying next is nuking my mozilla profile and re-installing and seeing if there was something there. maybe I can at least save my bookmarks someplace.