Firefox uses an extensionless browser cache, so they won't be checked normally by avast, which is checking files by their file extension. My setup doesn't scan firefox 1.0, but I'm not overly concerned.

The fact that the cached files don't have extensions it would be difficult for them to be used maliciously, any action would have a filename with an extension it is looking to open/run and that filename (and extension combination) doesn't exist.
Check your cache location C:\Documents and Settings\YourUserName\Application Data\Mozilla\Firefox\Profiles\default.3tu\Cache and see if you can recognise/identify any files.