AdBlock Plus is in constant development, if the almost weekly updates (not counting list of blocked URLs) are any indication. It complements the popup blocker nicely, speeds up browsing a great deal, and has NO equivalent anywhere else (the Google Chrome version merely hides ads, but Chrome still downloads the content; between Chrome+AdBlock and Firefox+AdBlock, the speed contest isn't one: Firefox browses faster, forget about Javascript speed)
NoScript also is updated often. These are mostly bug fixes, but also try to take into account all the contexts in which an unauthorized script may try to inject itself into an authorized content. This may be a bit tricky to implement, but results are here: it thwarts most script injections techniques used.
Web Developer Toolbar also is a useful extension; eventhough it is targeted at developers (as FireBug, the ultimate HTML+Javascript+CSS+DOM debugger, is), its ability to save 'generated source code' may allow someone to save a full dynamic web page in the exact state it is at the moment is unparalleled.