Are you receiving a message that A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.
If so, then try starting Firefox in Safe Mode – In Windows XP choose Start, All Programs, Mozilla Firefox, Mozilla Firefox (Safe mode)
You don’t want to disable or delete nsBlocklistService.js because this file defends you from badware or poorly written ad-ons.
The problem could be related to a new extension and was the problem and solution in my case. I got lucky because right after that I received a message like that from nsBlocklistService.js but this time it was Script: chrom://firebug/content/xpcom.js:16
Hope that helps!