Script Shield usually works for me (WinXP Pro SP3, Firefox 11.0 now). However yesterday morning after powering up my machine, when Avast VPS automatically updated I clicked on the popup to view the Avast updates page. I then proceeded to visit the forums. That always yields Script Shield activity, except this one time. When I checked Script Shield it was 0/0. Closing Firefox 10.0.2 (was before Firefox update was released) and restarting it jolted Script Shield into working, and it has ever since. Right now it's at 1025/0 (about 4 hours run time) and Firefox is the only browser I've had open.
Why Script Shield stopped working and why restarting Firefox got it working again is a mystery.