Firefox 9, Firebug, and YSlow had some compatibility issues which caused YSlow not to work for many users. The good news is that the Firebug, and YSlow extensions have been updated and now if you are using
Firebug 1.9.1 and
YSlow 3.0.9 then YSlow will work “Out of the Box” with Firefox 10.
If you had these extensions previously disabled make sure re-enable them and you should be good to go.