Well, there's plugins and then there are extensions. They're two different things. Plugins are generally for much larger things like Java or Flash, while extensions add some kind of functionality to Firefox.
But you can disable plugins and extensions, so you can go in, disable all of them, try some of your regular sites and see how things work.
I would strongly recommend keeping AdBlock Plus and it's two companion extensions Element Hiding Helper and the latest one that helps block sites from launching popups when you click on something. NoScript is also one of those I'd consider to be essential, and can really take the place of extensions like Flash Block. Those two I would consider essential, NoScript more than the rest since it improves the overall security of the browser, the rest are probably more in the nice to have category.