what you see is not the actual download being scanned by the web shield, it's the caching of it in Firefox. Keep your download link in your download manager. Download again later directly from the DM when the browser is closed, it won't be scanned.
The webshield might scan on port 80, it only works with browsers (what's commonly called browsers) and not with download managers. Doesn't matter whether DMs are browsers or not, only a few "browsers" are compatible with Avast webshield.