Normalize "updateDate" and "installDate" fields for Firefox extensions information

[jjsendor]

Firefox extensions information retrieved from extensions.json file in the user's profile contains fields with timestamp data, e.g. updateDate and installDate.
They should be normalized like the other timestamps retrieved by OSXCollector to a human-readable format.

It appears that the timestamp format used there does not fall into any of the four formats already recognized by the OSXCollector timestamp normalization function (seconds since 2001, seconds since epoch, microseconds since epoch and microseconds since 1601).