If you are web developer then there are two plugins for Firefox that are a must.
The first is the Web Developers Toolbar.
The other great plugin that I really love is Firebug.
There's a great part of this plugin that I use, which is the Inspect Tool. Have you ever made a site and wondered why your div is a few extra pixels wide? With Firebug you can identify the problem with ease. It displays the width, height, padding, border, margin and offset of each element in the page so you can see why your page isn't displaying properly. Plus you can edit pages on the fly so you can see how they look straight away.