Norton Internet Security should have an addon/extension in Firefox and other popular browsers that allows you to right click and select "Send to Norton" to submit a download URL and have Norton's servers download and scan the file for threats and report back the results. This could help protect your computer from worms that would bypass Norton's Download Insight and immediately infect your system.
This service would be similar to the Dr. Web and VirusTotal's VTzilla Firefox extension/addon.
Info on Dr. Web http://lifehacker.com/295612/scan-files-for-viruses-before-your-download-with-drweb#comments
Dr. Web official site http://www.drweb.com/
Dr. Web Firefox extension URL https://addons.mozilla.org/en-US/firefox/addon/drweb-anti-virus-link-checker/
Info on VirusTotal's VTzilla http://www.virustotal.com/advanced.html#browser-addons and http://lifehacker.com/5614928
/vtzilla-scans-files-for-malware-before-you-download-them
VirusTotal official site http://www.virustotal.com/
VirusTotal's VTzilla firefox extension URL http://www.virustotal.com/advanced.html#browser-addons