I cant find the URL where I found this tweak but it is a very useful one to do. There is a config setting called **browser.cache.disk.capacity **within firefox if you drop the value down from 50000 to 15000 this speeds up the load time and you should also find that firefox should be a little more friendly on the system memory.
FYI, if your not sure how to find the config settings open up firefox and then browse to the following ‘about:config’ minus the quotes.