and is irreversible. Note that this doesn't turn off download history; it only clears the currently stored records. New download records accrue as you start new downloads. If the Clear Download History Now button is unavailable, your download history is already empty. You can clear individual download records from the Download Manager window. For more on the Download Manager as well as anything else related to how Firefox downloads anything off the Internet, turn to Chapter 11. Sweeping up your cookie crumbs Billions of different Web sites exist, each serving a different purpose. Although Firefox can help you remember aspects that are common to many of them - such as the concept of a username and password - it is impossible for Firefox to remember the information necessary for each Web site genre. For example, what about a shopping site that needs to remember what's in your shopping cart as you navigate the digital aisles? What about an online newspaper that wants to remember that you like the sports page? If the other Firefox developers and I spent our time designing a custom "memory" for anything a Web site might need to store, we wouldn't have time for anything else. Clearly, then, Web sites need their own (generic) way to remember information about you, and the mechanism for doing this is called a cookie. A Web site can leave a cookie - a brief bit of information - on your computer, and your browser sends it back to the site when you return to that site. It's as if the Web site is temporarily storing a tiny memory on your computer. To prevent your computer from getting cluttered up with these memories, most Web sites owners give their cookies expiration dates. By default, Firefox - like virtually all other browsers - allows all Web sites to set cookies and keeps them until they expire. But Firefox gives you extensive control over which cookies are set and for how long they are kept. Warning You can impose more restrictions on which sites are allowed to set cookies, with the understanding that certain Web sites might not function properly without them. To configure or clear cookies, follow the directions in the section "Working One-on-One with Your Data," earlier in this chapter, to get to the Cookies tab of the Options window. From there, you can do any of the following: To prevent all Web sites from setting a cookie, deselect the Allow Sites to Set Cookies check box. You can specify an exception to this rule by clicking the Exceptions button, entering a Web address, and clicking Allow. To undo this decision later, return to the Exceptions window, select the Web site in the list, and click Remove. I cover the Exceptions window in more detail in Chapter 16. To prevent certain Web sites from setting a cookie, leave the Allow Sites to Set Cookies check box selected, but click the Exceptions button, enter the address of a Web site, and then click the Block button. To undo this decision later, return to the Exceptions window, select the Web site in the list, and click Remove. To prevent so-called affiliates from setting cookies, select the For the Originating Site Only check box. An affiliate is a Web site that lives within another Web site but is not actually part of it. Though you don't realize it, most of the Web sites you visit every day contain affiliates in the form of advertising. Most of the banner ads you see, for example, are not actually part of the Web site that contains them, but are generated by a third-party advertising firm. Because these are another form of Web sites, they can also set their own cookies, and most of these cookies are used for undesirable purposes such as tracking your viewing habits as you move among other Web sites in the ad's network. Tip Allowing only the originating site - and not affiliates - to set cookies is a good way to ensure that Web sites continue to work while preventing affiliate advertisers from storing cookies.