Ever wonder what is in your cache? Type ‘about:cache‘ in the address bar. This will bring on stats on your cache, then at the bottom of that page is a link ‘List Cache Entries’ which may take a bit too load if your cache is large. This page will list all your cache entries. Also, you can directly access the cache entries page by typing in the address bar ‘about:cache?device=disk‘. Now, keep in mind Firefox uses two caches, disk and memory. To view your memory cache, type in the address bar ‘about:cache?device=memory‘.
Also, there is the CacheViewer Extension which allows you to view the contents of both the disk and memory caches. It will also provide a GUI preview of the content, besides showing how much of your disk space and memory is being used up by the cache. Once installed, CacheViewer will be available via your Tools menu.