error message

I get the following error message by using Yslow.

=============================
FirebugContext.yslowContext is undefined
jquery-1.2.6.pack.js()()jquery-1...6.pack.js (line 11)
[Break on this error] eval(function(p,a,c,k,e,r){e=function(c)...r|serialize|pixelLeft'.split('|'),0,{}))
===============================

Could anyone tell me how to fix it?

Thanks in advance.