You think you only run a handful of extensions until you stop and check:
Adblock Plus : Absolutely required; should be built-in.
Aging Tabs : Colors tabs according to age. Might remove this one.
All-in-One Sidebar : For the Opera enthusiast. It's a bit much, and might be removed, but it does look pretty useful.
Anonymization Toolbar : Goodies for the paranoid.
Clipmarks : Select portions of a web page to print, save, or stash online. Sounds great, but it's surprising how rarely one uses this.
CustomizeGoogle : Control all sorts of stuff that "do no evil" Google does behind your back.
del.icio.us Bookmarks : Yeh, del.icio.us is annoyingly web 2.0 and lifehackery-smug, but it's still useful for sharing bookmarks across multiple machines.
Dictionary Search : Select text and look it up in a dictionary (opens in a new tab). Up two four dictionaries can be configured: very nice.
Distrust : A paranoid mode for Firefox; when exiting Distrust Mode, all history, cache, and cookies collected since turning on Distrust Mode are deleted.
DownThemAll : Download manager for repetitious tasks like saving all the images or videos in a page.
Extended Cookie Manager : Status bar icon for allowing/preventing cookies from a site.
Extended Statusbar : Status bar text that shows why a page is taking so long to load.
Fasterfox : Modifies ffox to make it behave more aggressively and load pages faster.
Firebug : Turns ffox into a debugger for web development.
Flashblock : Another required add-on: replaces Flash applets with a play button, so they do not start automatically.
Flat Bookmark Editing : More from the "why didn't Mozilla think of this" dept: this modified the Bookmark Manager so you can edit bookmark properties without having to open a separate dialog.
Grab and Drag : Replaces the cursor with an Acrobat Reader-style hand so web pages can be "grabbed" and scrolled like PDFs.
iDND : Required anti-spoofing add-on that shows an icon on the statusbar which is either green (traditional domain name) or blue (international and potentially-spoofed domain name).
iMacros : Automate web tasks with macros. Haven't played with this enough yet to recommend it whole-heartedly.
Image Zoom : Straightforward; allows you to zoom images.
Load Time Analyzer : A simple toolbar for measuring the performance of your web pages.
Map+ : Context menu item that looks up selected text (presumably an address) on Yahoo Maps.
PopupMaster : Statusbar icon for managing popup windows. Who really likes popups, anyways?
ShowIP : Shows the IP address of the current page on the statusbar, with options for doing a whois lookup and so forth.
TabScope : Shows a mini-preview of tabs on mouseover, with navigation (forward, back, refresh, close) buttons. Can slow down navigation if you have many old (i.e. cached to disk) tabs open.
TamperData : Very useful Sidebar window for viewing and modifying CGI parametersand HTTP headers.
Torbutton : Statusbar icon to toggle Tor for the paranoid. Requires tor and iproxy to be installed.
Torrent Finder Toolbar : If you like torrents (of course no-one does), you'll love this.
TrackMeNot : A curious tool for the paranoid that attempts to fight search engine profiling by generating random search requests from your browser. Incorrect settings will slow down your browsing.
URL Link : Context menu item that opens selected text (presumably a URL) in a new tab. Good for plaintext files and websites who forgot to wrap their links in A tags.
UrlParams : Similar to TamperData. This shows the CGI parameters (GET and POST) for the current page in the sidebar, while TamperData is used to set parameters for pages you are about to surf to. Run them side-by-side to see the difference.
View Source Chart : Displays the HTML source of a web page in easy-to-read 'chart' form. Has to be seen to be believed.
Web Developer : The most useful add-on for dealing with annoying sites or developing web pages. Provides a toolbar with menus for manipulating web page images, stylesheets, cookies and forms, as well as for viewing details of the underlying HTML. Highly recommended.
Zotero : Tool for academics, though it is useful for anyone doing online research. Provides a catalogue for saving and annotating (instead of just bookmarking) web pages, emails, files, etc.
Obviously this selection will be overkill for most people (how many paranoid developers with a research background are out there, anyways?). If for nothing else, install all of these to really test how poorly ffox can be made to run on your system.