First of all if you are not using Firefox, you should start using it now. Its faster than IE and has tons of free plugins and extensions.
If you are web developer or web designer. You should think about using some of the plugins mentioned here. Trust me, you will love it and your work will speed up.
2) Pixel Perfect
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML. This can be used along with firebug
This is very neat plugin. It will capture web page or any part of it as an image.
This is PHP programmers delight. FirePHP enables you to log to your Firebug Console using a simple PHP method call. All data is sent via response headers and will not interfere with the content on your page. This can be good way to debug your code.
As the name suggests, this plugin can give you vital information on whats going wrong with your web page. It will find why its slow. As users hate slow responding websites, this plugin is must have to make sure your website loads faster.
When you install plugins make sure you only install the ones you will use. Too many plugins will slow down your browser.