Firebug is a free, cross-platform and open source add-on for the Mozilla Firefox web browser, designed to allow web developers and web designers to easily and quickly find flaws in the source code of their website(s).
Getting started with Firebug
This Firefox add-on is easy to install, with a single click of a mouse, and requires no restart of the web browser. To open it and get started with web debugging, simply press the Firebug button from the main toolbar. The Firebug’s interface will open at the bottom of the window.
Firebug in interactive, which means that you will be able to click an element on the web page to inspect it. An error console is also implemented in the user interface, as well as quick access to the add-on’s settings.
The Firebug toolbar button
After installation, the add-on will automatically place a button on the main toolbar, allowing the user to quickly activate it, as well as to configure various essential settings, such as where to place the user interface (top, bottom, detached, left or right). The button also displays the total number of errors found on the current web page.