an obvious place to start would be the FIREFOX plugins top downloads and select the best individual plugins for every category.
for my money, I can't live without any of:
* CoolIris -- or at least some other image/video viewer to make ugly webpages much sexier (this maybe not for all the cardigan-wearing-types, but it is very popular) -- it is on every other major browser, so I think it's time the developers look at Google Chrome, or someone borrows the format.
* Sxipper -- or a similar webform/password filler -- the web is sooo full of form-filling crap these days, it's just intolerable without some help, especially if like me you use a large number of different websites (there are those who use only a handful of sites, this isn't for you).
* localised browser dictionary -- to highlight misspelt words in your local language (everyone should really have this as a way to improve degenerating language standards, but also a handy check for unusual/difficult words and late-night blogging sessions

* Flash and AdBlock are good, worthwhile additions too, along with flexible controls on JavaScript/Java including whitelisting.