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.
Many people swear by Spaces, a feature on the Mac that allows you to have multiple desktops, and have applications “assigned” to each desktop. I think it’s a fantastic idea too, but using it together with multiple monitors just creates a bad experience.
The current problem with it now, in my opinion, is that Spaces takes all your monitors as one desktop. So if you switch from Space 1 to Space 2, all your monitor switch together. That makes using your spaces much harder to use. Ideally, each monitor should be able to have their own independent space, so that you can have better control of what goes where.
Until Apple incorporates this functionality, I won’t be using Spaces to its full potential. As such, I’ve resorted to using Spaces in a pretty “dumbed” down way while my Mac is plugged to my external LCD. Instead of a fancy 4×4 or 3×2 matrix or something, I only have 2 Spaces created. The first space is my “working” area where everything opens in by default, and my second space is my “all others” area, where I can throw things that I need open, but don’t have to refer to as much. These can be things like my FTP windows, my Mail window, my Adium, my Tweet Deck etc. Everything else has to be in the first space because I find that I work between them all the time (e.g. moving files, copying text, referring to documents, and so on).