You can't clear everything Browser will always have a cache to use to load and that for it's operation not any related sites. If you had web cookies 32kb would be far bigger in size.

It has always gone to 0 every day when I clear the cache. That one day it didnt and now it has gone back to showing 0. Wasnt sure if there was a problem in there or if firefox was holding something in the cache.

You can see what is in the cache by visiting about:cache in the address bar.