Firefox is fantastic, but you can make it better. Add some of these extensions by visiting the site & clicking the install link. If you are using firefox, the extension will automatically install and be ready to use after you restart your browser.
Tab Mix Plus: Allows you to control how you browse by opening tabs for events you specify. This gives you full control over what happens when you browse, and when new windows get opened, recycled, or new tabs get opened.
IE View: This adds an item to your right click menu that opens the current page in Internet Explorer. Some poorly designed pages require the use of internet explorer, and as a web developer I am constantly checking my pages in both browsers.
del.icio.us extension: If you use del.icio.us to keep track of sites & bookmarks, you can replace your firefox bookmarks completely with this extension. Its also a great way to keep bookmarks synced between computers if you have more than one. It gives you the option of displaying del.icio.us bookmarks in a toolbar.
Web Developer Extension: If you don't do web design, you don't need this. However, if you do, you need this more than you can imagine. My favorite feature is the "edit CSS" which allows you to play with a page's CSS in real time to see the changes. Other useful features are outlining block level elements, and turning off images.
DOM Inspector: This goes great with the web developer toolbar for anyone dealing with web design. It allows you to examine any element on a website and see what its parents are and all of its properties
Google Toolbar: This allows Easy searching of google news, froogle, google, images, and groups. It also has a very useful spell checker for web forms and displays search terms at the top of the page. You can click on them to jump to the point in the page where the term appears, which is useful for wordy pages.
BugMeNot: When you get to a site that requires free registration, you can right click on the login field and select bugmenot. This anonymously logs you in saving time and privacy.
Adsense Notifier: If you use google adsense to monetize your website, this statusbar extension will show you how much money you made today and the number of views and clicks.
FireGPG: This extension provides an interface to encrypt, decrypt, sign or verify the signature of text in any web page using GnuPG. This allows you to send emails securely.