Install Greasemonkey add on into firefox, then download the following user scripts.
Numbered Google SERP results
Place a number on the Google SERP window
Numbered Yahoo SERP results
Place a number on the Yahoo SERP window
Numbered MSN Live SERP results
Place a number on the MSN Live SERP window
You might also be interested on the following scripts below
Google 100
Show 100 results at a time in the Google SERP window.
For some reason, the numbers are not showing up for me. Anyone else seeing the same thing?