Xmarks absolutely - the best add-on I ever found. Keeps my laptop and my desktop synchronised.
NoScript is essential to keep the crawlers out of your browser.
AdBlockPlus - yes!
WebOf Trust - warns if you're being taken to a dodgy site. It triggers on my PC several times a week, usually when some apparently innocuous site is linking me to one that is less so.
Forecastfox - simple, up-to-date weather info in the bottom toolbar.
British English Dictionary - used it in the past, not bothered lately.
I found the ones that stick tab bars and the like around the screen take up space that would be better used for displaying websites without needing to scroll so much.
I used to have dozens of add-ons. When I switched the laptop to Win 7, I cleared them out and reduced it to the list above. I don't know if Firefox runs faster as a result.