Temporary Internet files are known as the "cache" in Firefox.

Type about:cache in the URL bar and hit Enter.
Memory cache device - List Cache entries

You can find the location of the cache on the about:cache page (open that page via the location bar like a website).
Only large image and media files are stored as individual files on disk.
Other files are stored in a few large container files (_CACHE_XXX_) in the cache folder and not as separate files on disk.
The individual files on disk have random names without a file extension for security reasons: you can't open an application or start a downloaded virus inadvertently by clicking a file in the cache folder.

You can look at this extension for easier access to files in the cache.

• Cache Viewer: https://addons.mozilla.org/firefox/addon/2489

XP: C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla\Firefox\Profiles\<profile>\
VISTA: C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\