Situation where FFD < BFD
Isn't First/Best Fit Decreasing always considered offline (requiring sorting the input assumed unsorted)?
Anyway, let me assume "performs better" to mean uses less bins.
I find it annoyingly hard to try to find an example using a minimum number of items, of item values and minimum item values - and to argue convincingly that it is minimal, but  


   try a bin size of 16 and items of 11, 6, 6, 3, 2, 2, 2. Bin contents after adding each item:
 - FF: 11 11,6 11,12 14,12 16,12 16,14 16,16
 - BF: 11 11,6 11,12 11,15 13,15 15,15 15,15,2