A great thing about Firebug is it gives you the option to add extra extensions and make it that even more indispensable. You can tailor it and mold it to your needs. I have listed fifteen of my favorites in this article.
XRefresh for Browser Refresh Automation
XRefresh for Browser Refresh Automation is a browser plugin which will refresh current web page due to file change in selected folders. This makes it possible to do live page editing with your favorite HTML/CSS editor. Typical usage scenario is for local web development on a machine with two monitors, the first monitor is for source code editing and the second one is fully dedicated to previewing the web page in Firefox.
YSlow for Firebug
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages. It grades web page based on one of three predefined ruleset or a user-defined ruleset. It offers suggestions for improving the page's performance, summarizes the page's components, displays statistics about the page, and provides tools for performance analysis.
Firefinder offers the functionality to, in a quick way, find HTML elements matching chosen CSS selector(s) or XPath expression. It allows you to instantly test your CSS selectors in the page while seeing the content at the same time, and matching elements will be highlighted.
FirePHP for AJAX Development
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. It is ideally suited for AJAX development where clean JSON and XML responses are required.
FireQuery for jQuery Development
With FireQuery, jQuery expressions are intelligently presented in the Firebug Console and the DOM inspector with the elements in the jQuery collections highlighted on hover. With jQuerify you can inject jQuery into any web page.
Drupal For Firebug
Drupal for Firebug extends the Firebug module to provide Drupal specific debugging and status messages. It allows users to see the processing of the node, form, and user objects and works with the Devel module to display SQL queries and execute PHP from the Firebug window.
FireAtlas – Asp.net Debugging
FireAtlas is an extension dedicated to ASP.NET AJAX debugging. This extension will provide you with these useful services when developing ASP.NET AJAX web sites: PageRequestManager events tracing; WebService calls tracing and inspection; Partial Update inspection within Firebug Net Panel; Application components listing.
CSS Usage is an extension which allows you to scan multiple pages of your site to see which CSS rules are actually used. Each time you press "Scan", the CSS files that are included in the current page are shown with the number of times the rules has been found applied on your page before it.
FireSymfony is a port of the Symfony Web Debug Toolbar as a Firebug extension. Besides the normal functionality of the Symfony Toolbar, it adds new features, like Database Query, One Click copy to clipboard, or Mouse Over Partial and Component Cache Highlighting.
Firepicker – Color Picker
Firepicker adds a small color picker dialog when editing CSS rules in Firebug. The addon scans for values that can be parsed as CSS colors (#abc, rgb(1,2,3), blue, red etc.) and displays them in a drop-down list right below the firebug's CSS editor box. Clicking on the color value in the drop-down brings up a color picker dialog.
Firepath – Edit, Inspect and Generate XPath Expressions
FireXPath is a Firebug extension that adds a development tool to edit, inspect and generate XPath expressions. With this extension you can edit with auto-completion (using TAB or up and down arrows), you can also evaluate the expression on HTML or any XML documents and display the resulting evaluations in a Firebug-like DOM tree.
Inline Code Finder
SenSEO analyzes web pages and tells you how good they fulfill on-page Search Engine optimization criteria. SenSEO gives you an overview of SEO-important web page components, indepth analysis of on-page SEO criterias and offers the correct usage of Document-Title, Meta-Description and Meta-Keywords.
CodeBurner – Reference Tool for Web Developers
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Pixel Perfect is a useful extension that allows web developers and designers to easily overlay a web composition over the top of the developed HTML. By toggling the composition on and off, the developer can visually see how many pixels they are off in development.