My Norton Corporate caught and quarantined a Trojan Downloader virus, but it can't repair the infected file:

C:\Documents and Settings\Administrator\Local Settings\Application Data\Mozilla\Firefox\Profiles\ukawnw5u.default\Cache\_CACHE_002_

Is this my browser cache? Is Firefox corrupted? How can I go in and delete this myself?

You should be able to go into Norton's quarantine section, and delete the quarantined file.
As long as it's quarantined, it doesn't have access to your system.