If you work with cookies and you need to edit your cookies or delete only some cookies fast you can use Add N Edit Cookies, a firefox add-on. Thing is a "cookie editor".

A nice firefox add-on i have so i can immediately judge how much traffic a website has and how important it is, is search status. I got so used to it I find it hard to browse without it.
From the description:

Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and more.

If you want to see php errors in firebug here is how you do it: <?php
 
/**
* class: Debugger
* author: <a href="http://doru.serveblog.net">Moisa Doru</a>
* contact: <a href="mailto:moisadoru@gmail.com">moisadoru@gmail.com</a>
* info: this class displays PHP errors in Firebug; works only in PHP5, tested in PHP 5.2.0
* usage: <strong>include_once("debugger.php")</strong> as the very first include();
* licence: free as in free beer ;)
*/
 

JSView is a nice firefox addon that let's you see the source of external js files for example, easy.
From the description of the addon:

Another useful firefox add-on: yslow. It shows the loading time of a site and can help with figuring out ways for making it faster. Also a must have firefox add-on for developers.

This is the greatest tool for web developing I ever came across: the firebug add-on for firefox. I don't remember how debuging html for instance was done before it and I don't want to.