There is an enormous list of possible add-ins for Firefox. I insist on NoScript, to protect against nasties. Adblock is useful. I have a British English dictionary for spellchecking, and Forecastfox which gives today's and the next two days' weather for "Home airfield" in the toolbar.
The plugins that are essential are Java (for all those clever pages) and often QuickTime (for the alternative breed).
Once NoScript has learned the sites you use regularly (and trust), it's not a problem. For the first few days, it's a bit like a first install of ZoneAlarm.