If you work with many websites, or want to keep track of many different websites, and would like to do that simultaneously, then the default Firefox probably wouldn’t cut it for you. Safari has live bookmarks that may do what you want, but in my personal opinion, these following Firefox add-ons do a much better job.
Firefox Splitter (Formerly Split Browser)
Firefox Splitter is an add-on that allows you to browse with multiple tabs open at once in front of you (instead of having just one tab open in the foreground). This would be really useful to watch a video while web browsing, or allowing you to monitor your stock portfolio, or the latest news while you surf the net in another tab.
Showcase allows you to manage your current Firefox tabs and windows by showing them as thumbnails in a single window, tab or in a sidebar.
Made by the same people that gave you Showcase, µWorks. Speed Dial gives you the ability to have a home page or tab where you can put your favorite sites, and show a (live) screenshot of every website.