I was having problems with Youtube, using older versions of Safari and Firefox under Mac OS 10.3.

Firefox would bring up the YouTube page, but then produce an error message. Sometimes I could get things to work by clicking on one of the "related" links, then reversing back to the first page.

Solved problems (so far, so good) by upgrading to OS 10.4, and installing the appropriate Safari and Firefox versions (I seem to remember a Flash update was also required).

Ross