About XPIs, and components
Mozilla is like a mini-operating system, using the Gecko Runtime Environment (GRE). Each component is built on the Cross-platform Component Object Model (XPCOM). This means that you can add components to your existing installation, and reinstall components that are already installed. When you run the Mozilla installer, it will install the Gecko Runtime Environment, then install each component you've selected. The extension for each component is XPI, which stands for Cross-Platform Installer. Every time you wish to add on to your installation, all you need to do is open the XPI file in Mozilla. For instance, Mozilla makes all the components available on their FTP server, so if you want to reinstall the browser, you can go to the FTP server and click on browser.xpi. If you only have browser installed and want to add the mailnews component, you just need to open mail.xpi in the Mozilla browser. To install an extension, or a theme, it will be available in the form of an XPI file.
On the FTP server, you'll see:
A Mozilla profile is information that is specific to a user, such as preferences, bookmarks, mail and newsgroup accounts, address books, cookies, passwords, etc. Profiles are stored in a separate location from the directory in which you installed Mozilla. This keeps the information separate; so if/when you decide to upgrade, uninstalling Mozilla will not touch your profile data. When you first install Mozilla, a new profile named default is automatically created. Using the Profile Manager, you can create, delete, and rename profiles in any location you wish. Each profile registry is specific to each operating system account; so if you're using Windows XP, profiles for other XP users will not appear in your Profile Manager, and yours will not appear anywhere else. If you have more than one profile in your list, Mozilla will ask you which profile you want to use when you start. See the Profile FAQ for more info.
On this site, you may see references to Chrome folders. The chrome is that part of the application window that lies outside of a window's content area. Toolbars, menu bars, progress bars, and window title bars are all examples of elements that are typically part of the chrome. The chrome itself is not native to any operating system, so it is very customizable. This is why you can change the skin of the entire program.