i have firefox 5, since yesterday youtube doesn't work anymore, it starts but stops immediately and the black screen is bigger than before

Check if clearing the cache and deleting cookies for youtube.com solves your problem:

1. How to clear the Firefox cache
2. Clear cookies and site data in Firefox

I have been following the instructions but Youtube still has the same problem. It starts for a few seconds, then the screen gets bigger and it stops showing the pictures...