Every browser, including Firefox, has a place where they store bits and pieces of websites that you visit - images, ads, scripts, etc. - so that the next time you visit that particular site, you will see the page load quickly. This is called a cache, and Firefox gives you a coupla options here. As we did before, navigate to Tools, Options, Privacy, and then Cache.
The general rule of thumb on the cache setting is to have it no larger than 60 MB (that would be 60,000 KB), unless you have an immensely huge hard drive with lots of memory. This is just a good general setting. I have it set for 60 MB on my main machine, and 50 MB on my laptop, since my laptop is kind of a memory weenie (sorry, Lappy!).
One more thing about cache - if you for some reason want to get rid of everything, you can click Clear All there at the bottom of the window. Every single thing you've been doing on the Web will be gone if you do this - history, cache, cookies, passwords, etc. - so consider carefully before hitting the confirm button. And that's pretty much it for cache settings. You can get really technical here again, just like everything else we've gone through so far, but these settings are just fine for the general user.