My trick of turning a link like this: https://www.youtube.com/watch?v=wUcdb6xHybo

Into this:

https://www.youtube.com/v/wUcdb6xHybo

for playback on FF2 has recently sometimes not worked for some video's.  I still get a full-screen preview frame and all player controls are there, but when I hit play I get a screen full of static with the message "watch this video on youtube.  Playback on other websites has been disabled by the video owner".  Which is strange, since I am playing it on youtube.  Same thing happens on Opera 12.02  (but I can play the original un-modified video URL on Opera 12.02).  So if anyone knows why the modified URL is causing youtube to throw this error message and refuse to play the video, let me know...
 

/v/ link is equal to embedding a video on an another website. The uploader can set so that embedding a video is disabled, and the video will not open under the /v/ format.

Please read the following link to understand the technical background behind the "problem".

https://stackoverflow.com/questions/13407482/youtube-embed-gives-restricted-from-playback-on-certain-sites-error-despite-ap

 

As MrMateczko said, embedding may be disabled by the uploader.