I cannot live (ok, browse) without Firefox. One of the reason is that there are so many usable extensions. Here’s my list, and most of them could be found on Firefox’s update site. Only links of those extensions which could not be found there are indicated here.

Must-Have

• ieview
  Launch IE for viewed page directly from within Firefox. Also it can remember which page (when viewed by Firefox) should IE be automatically launched. For web sites that can only work in IE, it is really very handy!
• adblock
  Block partial content with wild card setting. Useful for AD prevention.
• autohide
  Make Firefox full screen truly full of screen. Help you hide the toolbar as well. Must-have for those using Firefox as presentation tool.
• spellbound
  Spell checking any text field or text area within Firefox. No need for any external dictionary or spell checker any more.
• imagezoom
  Add zoom function for images in Firefox. Very useful when image size does not fit very well.
• outsidebar
  Do you hate the expanding of sidebar to push the window to the right? It expands sidebars to the left to help keep your window static.
• disable targets for downloads
  Prevents an unnecessary blank page opened when clicking a download link.
• searchpluginhack
  Ever wondered how to remove an installed search engines? It can help you by providing a simple right click directly on the search box dropdown list.
• resizable-textarea
  Ever complained about the too-small textarea when editing on the web? It can resize textareas to your needs.
• pdfdownload
  Allow to choose how to handle a PDF file link (download, view it within Firefox, or view it externally).
• noscript
  Help control the usage (allowed or not allowed) of JavaScript (by site).
• all-in-one gestures
  Execute common commands via mouse gestures.
• linky
  Open/download/validate links and pictures in tabs or windows.

Usable

• forecastfox
  Shows weather forecast in the status bar.
• greasemonkey
  User script manager for Firefox. Required by many other useful user scripts.
• mozcc
  Shows embedded Creative Commons licenses of current pages.
• better search
  Enhance various search engine (search result page), including Google, A9, MSN, Yahoo, del.icio.us ...
• customizegoogle
  Enhance Google search result page and removes ads and spam.
• Merriam-Webster search box
  Merriam-Webster search engine to Firebox search box.

Web Mail

• gmail s/mime
  Add function of sending/receiving encrypted S/Mime messages in GMail.
• gmail delete button
  Add a delete button to GMail. Requires greasemonkey installed.
• webmailcompose
  Directly launch various web mail compose window for mailto: link.

Developer

• colorzilla
  Advanced eyedropper, color picker, page zoomer, and other colorful goodies.
• web color names
  Let you select color by name or by hex number.
• measureit
  Draw a ruler directly within Firefox to get the pixel width and height of any element on a web page.
• editcss
  Let you modify the currently used CSS content on the fly (in Firefox, not on the server). Very handy when you are experimenting different styles.
• aardvark
  Let you manipulate(hide/view/view source) pages by block. Very helpful for working with CSS styles.
• view formatted source
  View source inline or in a formatted (colored syntax) form.
• view cookies
  Add a cookies tab to view page info dialog to check used cookies for current page.
• add’n’edit cookies
  Allow you to add/edit session and saved cookies.
• web developer
  Add a menu and a toolbar with various web developer tools.
• javascript debugger
  Provide a JavaScript debugger for Firefox.
• curisecontrol monitor
  Show a CruiseControl monitor green/red light in the status bar.