I like that they now have 'about:config' (type that into your location box), which allows you to edit all kinds of fun stuff. But that's really a basic way of doing it. There are two add-ons that I really like, that I think add a lot to Mozilla. The first is MozTweak.
MozTweak adds a new set of panels in preferences that allow you to change many of the 'hidden' preferences in Mozilla in a convenient way. One of the things I do is turn off the http_referer header unless it is something I clicked on - so it isn't sent with image requests. That defeats many web trackers, adware, etc. You can turn it off completely if you wish. You can also do things like force frame resizability, which always shows borders between frames and lets you resize them - no matter what dumb sizes the author used.
Preferential is a bit like about:config, but it has a better UI and most of the settings are commented to give you an idea as to what they do. They're also grouped by functional area, which makes things easier.
I've also just started playing with the spellchecker add-on to Mozilla, which seems like a neat thing to have.
LiveHTTPHeaders is a geek toy - it lets you see the HTTP headers, and you can even edit request headers and 'replay' a connection. Nifty for testing things.
I tried the OptiMoz mouse gestures, since I've seen people talking about how cool mouse gestures are in Opera, and with OptiMoz, Mozilla - but I don't get it. I just couldn't get into using them, I kept accidentally triggering things.
So, anyone else have any favorite Mozilla add-ons and/or nifty preference settings?
BTW, if you haven't tried Wolfenstein5k, check it out.