Privacy & Security
Nice filters to block ads, hide text ads and space taken by ads, and disable stat counters.
Design
Handy toolbar. Among many other things, it allows you to see the tables, tables cells and CSS elements of a page, enable/disable images or styles, see the page in smaller resolutions.
"Capture entire web page or take screenshot of only visible part of the web page" + "editing and annotation tools".
Customize the style of a page, a domain, all pages and Firefox by creating CSS styles.
Usability
Choose which menu you want to display in the menu bar. Replace text with images if you want to.
http://yellow5.us/firefox/osb/ choose which icon/text you want to display in the status bar and in which order.
"Change your bookmark icons!" Very useful if you want to remove text from your personal toolbar by using icons only.
In FF, unckeck 'Accept Cookies' and do not set cookies to be removed when browser is closed.
In CS Lite, refuse cookies globally and keep cookies till they expire. - Atomic Orange on 2010-04-23