One of the best part of Firefox is how you can extend it. While other programs allow you to extend them some, with Firefox the options are great. This is a list of some of the extensions that I use:
Web Developer and Firebug are a couple extensions that are useful if you are designing web pages. They allow you to see your web page with different options and to find out details of the structure of the page. The Web developer extension has options for disabling various features of the browser to see what your site will look like in those circumstances, provides information about the HTML structure and CSS of the web page and will have your page validated. Firebug also gives a detailed view of your HTML and CSS.
Quartz PDF Plugin: This extension will allow you to view PDF files directly within Firefox, without having to download or open up a separate program.
There are also many themes that one can use with Firefox. The one that I like to use on the Mac is iFox Smooth.