Extensions are essentially mini-programs that are written to add some kind of extra functionality or features to the browser. Extensions add a wide range of featuressome add icons to the toolbar while others add items to the context menus. Some actually run as powerful applications within Firefox. Some examples of extensions that run as applications include ChatZilla, Mozilla Calendar, and the Mozilla Amazon Browser.
Third-party extensions are written in the XPI (Cross Platform Installable File) format, which means they install as an add-on to the Firefox browser. In all cases, you must download the extension and install it in the browser before it can be used. Installed extensions are managed via Firefox's Extension Manager interface, discussed later in this chapter.
Like all software you might download and install, Firefox extensions could contain potential security vulnerabilities or other bugs (or even intentionally malicious features). Although no extensions have been reported to contain security vulnerabilities (and some even offer additional security and privacy features), you should always do some research and evaluate the extension you want to install, the author, and the site it originates from. Extension sites frequently offer information, ratings, and postings from users about their experience using the extension. Use these as a guide, and ask questions in newsgroups and forums if you have any questions before installing.