Try clearing the cache and deleting YouTube cookies, this can sometimes help with accessing a site that previously worked. For details see https://support.mozilla.com/kb/How+to+clear+the+cache and https://support.mozilla.com/kb/Deleting+cookies

Hello TonyE, i appreciate your soon reply. Unfortunately this solution does not works so i don´t know what else can i do because i have seen that many people has the same problem and yet can not solve it. Maybe is some kind of trouble within youtube.com. I do not know... i am desperate! thanks again !

The problem does seem to impact upon YouTube more than other sites.

For some people who posted on this site, they had to go a step further after deleting YouTube cookies and block YouTube from saving cookies. In the Tools menu select Options to open the Options window. Go to the Privacy panel and make sure it is set to "Use custom setting for history". Click the Exceptions button, type in http://www.youtube.com and click on Block

Another option is to disable hardware acceleration for Flash. To do that right-click on a Flash video, select Settings, go to the first tab of the settings for the option to enable/disable hardware acceleration for Flash. This is something recommended by YouTube for those having problems - http://www.google.com/support/youtube/bin/answer.py?&answer=1209379

Thanks, it seems to be working fine know after a couple of days ive deleted the cookies... looks weird and i dont realize what is the problem itself but its solved now. have a good day