If you have been using YSlow for client side website benchmarking and you upgraded to Firefox 3.5, you must have noticed that it's not working any more. I guess YSlow development died at some point - I noticed couple of bugs some months back, mailed developers but nothing happened.
A pretty good alternative to YSlow is Page Speed from Google- also FireBug addon Firefox extension.
I am missing couple of pieces of information which YSlow reported but it will do for now. It has couple of interesting features for css gurus. My frontend code guru colleague said that he doesn't believe them but but addon is definitely worth checking out. :)