I think before we start speculating we should get the version information requested.
Since the script shied was incorporated into the web shield there really is only one shield to check, the web shield. The real problem here is the inconsistency of the problem, if it truly were a bug I would expect it to be consistent and see many others reporting the issue.
The only other thing to check would be the avast internet security (AOS) add-on, it may be possible that there could be conflict with any other security based add-on in the OP's installation of firefox.