I just noticed since updating to Firefox 21 yesterday, Avast’s script shield has not scanned anything. If I remember correctly this has happened before on occasions with Firefox updates.
Just took a look at the Script Shield in my install. I’m running Firefox 21.0 and avast! 8.0.1489 and the Script Shield is working fine for me. Been using the computer for several hours now and the Script Shield has scanned more than 23000 scripts . . . . Perhaps a reboot of your computer or a repair of avast! would help . . . . ?
Script Shield working fine on my FF 21.0.
My system has been on all day (updated to FF21.0 yesterday I think and over 10,000 scripts scanned so far and about to shut down now.
Strange, it works with IE and Chrome. But not with the new version of Firefox.
Can’t understand why that is for you when we aren’t experiencing the same problem.
Do you have any weird/unusual add-ons or settings that might be blocking the script shield ?
Mine is reading 6415 scripts scanned and I haven’t had much time on-line today (electricity off for 7 hours today).
I just updated Avast to the latest version and now the script shield works with Firefox. 
Ah, I though you were using the latest version, so never considered that, should have checked your signature (which is now out of date) ;D