I found some more very useful extensions for FireBug (Thanks WebResourcesDepot who mentioned these):
Very similar to YSlow, it is an add-on by the Google team for evaluating the performance of web pages and, best of all, getting suggestions (based on "Web Performance Best Practices") on ways to improve them.
Page Speed automatically optimizes images & provides compressed ones foruploading later.
In order to easily find out how a fresh element will look with the design in general, Pixel Perfect enables web developers and designers to overlay a web composition over top of the developed HTML.
It is possible to toggle on/off any number of design elements to see if they fit well or not. And, with the opacity option, HTML below the composition can be viewed.
CodeBurner offers a reference panel with a search tool for looking up HTML elements, attributes, and CSS properties.
It also provides shortcuts to view information or code samples about a selected item.
With the help of FirePHP, it is possible to log to the Firebug console by a simple PHP method call.
Data is sent via response headers & won’t interfere with the content on the page.
For any developer who got used to the Firebug interface , this extension is great for PHP debugging.
FireRainbow adds a better syntax highlighting support to Firebug & has theming support.
It adds a change monitor recording every change made, by Firebug and the website itself, to CSS and the DOM.
This provides a deeper look into the functionality of the website & helps keeping a record of the changes that were required to debug and tweak the page’s display.
It enables you to measure the loading times of webpages.
The extension has support for multiple pages & can clear caches (disk & memory) with a click for a healthier result.
Firecookie, is a cookie viewer/manager that works inside Firebug.
For a faster development, when coding an application using cookies, the panel eases viewing the cookies & their values, expire dates, etc.
It is also integrated with the Net panel & displays both sent and received cookies.
Firefinder offers a way to quickly finding HTML elements matching mentioned CSS selector(s) or XPath expression.
It allows you to instantly test CSS selectors in the page while seeing the content at the same time, and matching elements will be highlighted.
It also shows a count of how many matches there are & lists all the matching ones in a collapsible list in the Firefinder panel.
LiveCoder extends Firebug by allowing developers to look at the Firefox chrome.
It is especially valuable to developers building add-ons. LiveCoder’s GUI lets you examine and modify the DOM of any Firefox add-on.
For best results, use the Firefox browser..