You certainly can manually copy MP3s (or any other files for that matter) to your iPod once FireWire disk mode has been enabled in iPod Preferences. But, simply copying MP3s to iPod as a hard disk drive will not allow them to be played on iPod.
The iPod has a database on it that the iPod’s user interface uses to allow you to browse your MP3s by artist, album, genre, composer, etc. This database contains info from the MP3s’ ID3 tags, and it also contains the location of each MP3 on iPod’s HDD, like a directory.
And, these playable MP3s are stored as hidden files in special hidden folders on iPod’s disk.
Programs like iTunes on a Mac and the iPod-enabled version of MusicMatch Jukebox (MMJB), XPlay, or EphPod on a Windows PC update this database when they copy MP3s to the iPod, and they store them in these special hidden folders.