Here are a couple of FF Add-ons that I have been finding really useful:

FireFTP - great for using FTP as a tab in your browser window. Saves your connection profiles and makes it easy to do quick updates and previews with web development.

GMarks - If you like using Google Bookmarks instead of a social bookmarking site, this add-on makes it easy to organize your bookmark labels as folders. You'll definitely want to set it up to use the sidebar - this seems to work the best. Once you are up and going and have all of your bookmarks organized, it is simple to drag a link from your address bar directly into the sidebar folder you want it to appear in. Also has a nice search feature. 

 FireBug - Definitely worth a test drive if you still haven't tried this yet. This really eliminates the need for the old 'right-click-view-source-selection' technique in order to troubleshoot layout and CSS issues. Of course, it does a lot more than than, but I use it because it previews the source code and CSS really well.