It scans the http traffic. The firefox add-ons are local files and aren’t scanned by the web shield. NoScript only stops scripts from running, if that script is embedded in the page then that is a part of the actual page content and would be part of the http traffic coming down to the browser, that would be scanned.
AdBlock+ blocks ads, etc. so they wouldn’t actually be downloaded for display on the browser, so there would be nothing in the http traffic for the web shield to scan.
I have NoScript and AdBlock+ and no problems.