Firebug opens at the bottom of the browser or if you prefer it can be opened in a separate window. The advantage is that you can work directly in the browser, rather than from an external application where you have to save the changes and reload the page; Besides that it is very complete.
This add-on allows you to view the source code of a page, with the advantage that you can select any sector and easily locate the associated code, see its attributes and properties. You can edit the code, for example, the values of a certain property within the CSS file corresponding to a web page, and see reflected changes in the layout of the page, instantly.
It is also possible to inspect DOM objects and edit them on the fly; View the calls using Ajax (via GET or POST) and see the values that are obtained and their response time; And do many more things.
In turn, Firebug supports extensions, with the aim of adding more functionality and increase the possibilities of code debugging that is counted in the standard version. In the official page you can consult and download the available extensions.
FireBug 1.12.1 is the latest version, updated in August 2013, of this add-on for Firefox that you can download free. It was updated to support up to Firefox version 23 .