Suppose you’re in a library or using someone else’s computer. You don’t want them to see your form submissions or list of visited sites, right? That’s your business, not theirs.
Ctrl+Shift+Del and a dialog box appears, with some default items to clear checked, and others not. (Remember you can use Tab and Space to change the checkmarks.) Once you press Enter, you’ve just wiped out all of your history (and theirs too, for that matter). Bookmarks do stay.
(Under Tools | Options, there’s a Privacy tab. Sadly you have to use the mouse to select this tab. Once there, you can select the “Always clear my private data when I close Firefox” option, if you find yourself clearing private data frequently. This option is off by default.)