I have been using Firefox for almost a year and continue to be impressed by this product. I like the ability to add extensions - third-party applications that enable the user to customize and expand Firefox. My favorite extensions are:
All-In-One Sidebar - This one was recently installed based Ed Bott's list of favorite Firefox Extensions. It lets you view bookmarks, downloads, extensions, and more in a sidebar tab.
Autofill - Automatically fill out HTML forms or automatically highlight fields that can be filled out by Autofill.
Bookmark Backup - Create a backup of your bookmarks and more each time you exit Firefox.
BugMeNot - Get sick and tired of all those "free" news sites that require you to be a registered user? With BugMeNot you can retrieve a user name and password from public database at bugmenot.com. If you have the Autofill extension installed, BugMeNot will automatically fill in the user name and password. A warning: several news sites regularly check bugmenot.com for 'rogue' accounts and delete the accounts.
del.icio.us - Integrates Firefox with del.icio.us, the social bookmarks manager. A quick way to add pages to My del.icio.us bookmarks. Integrates into All-In-One Sidebar.
Forecastfox - Want to know current and future weather conditions? Then this is the extension for you. Totally configurable, you can put the weather in a toolbar, sidebar or status bar, choose which variables to display and for what cities. Weather information from Accuweather.
FoxyTunes - Lets you control your favorite media player without having to leave Firefox.
IE View - For sites that require IE, you can right-click on a link or on the page itself and open the URL in an IE window. You can also add pages to the "Always-in-IE sites" listing and Firefox will automatically open that page in IE.
Sage - A lightweight RSS and Atom feed aggregator. Integrates into All-In-One Sidebar.
Tab X - Add a small close button to each tab and remove the close button from the end of the tab bar.