Following are the Firefox extensions:
Those of you using the Firefox browser will want to check out the Firebug extension. Firebug allows you to pop up an inspection pane that contains information about the page in the browser. You can then click on elements and see the details of their formatting in the inspection window.
Firebug is a great time-saver for CSS work as you can not only view the styles at work but also edit them to see the impact on the page element. Firebug also eases work with JavaScript and, with the YSlow extension (a separate add-on you must install), you can even measure and benchmark page performance.
Learn more: https://addons.mozilla.org/en-US/firefox/addon/1843
Like Firebug, the Web Developer Extension is an add-on for the Firefox browser. Web Developer provides a wide variety of information about web pages as well as the ability to identify selectors and to edit active stylesheets. Web Developer also provides useful tools for forms and even links into different validation engines.
Whether you prefer Firebug or Web Developer is up to you. Firebug is faster and easier to use, but Web Developer provides a great deal of information. Both are installed in my Firefox browser.
Learn more: https://addons.mozilla.org/firefox/addon/60