Hmm normally Norton does this false positive with SeaMonkey for a long while now and not with any builds of Firefox.

Filename: xul.dll Threat name: Suspicious.Cloud.9.BFull Path: c:\program files\nightly\updated\xul.dll

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Details Unknown Community Usage, Unknown Age, Risk High

Origin Downloaded from

Unknown

Activity Actions performed: Actions performed: 1

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On computers as of Not Available Last Used 10/30/2014 at 8:34:07 AM Startup Item No Launched No

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Unknown It is unknown how many users in the Norton Community have used this file.

Unknown This file release is currently not known.

High This file risk is high.

Threat type: Heuristic Virus. Detection of a threat based on malware heuristics.

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Source: External Media

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File Actions

File: c:\program files\nightly\updated\xul.dll Removed ____________________________

File Thumbprint - SHA: 7f5831b05fcbe1e99e46ab712436c651d39c7d0bb5623229715bee224e47101f File Thumbprint - MD5: Not available This is what comes up with norton. As you can see, it directly points to the update.