Every time I have used FireFox it has seemed to be slower to me, even freshly downloaded. But it could just be a fluke.
For me, two main ones: a weather one that incroporates a current radar map over the UK, and secondly the godsend that is Zotero, which is basically a configurable referencing collection tool. I write a LOT of essays for my course, and with Zotero I can find journal and peer-reviewed articles and not have to worry about spending hours at the end searching for the same articles again, because I can get a quote to back my point up and then carry on searching without having to reference the article there and then, which breaks my concentration. It may be a little specialised compared to a lot of other peoples’ favourite plug-ins, but it’s incredible for me.
TL;DR: Zotero tracks the articles I’ve quoted for essays.
fair enough :s hrug: