If Developer Tools are a complete secret to you, you may have had difficulty finding them!
As well as a menu shortcut, you can access the Developer Tools easily either pressing "F12" in Windows or "Cmd" ⌘, "Option" ⌥ and "I" on the Mac, or by right clicking on the page and selecting “Inspect Element”.
In Safari, you will need to enable the Developer Tools by checking the option in Preferences > Advanced > Show Develop Menu first.
In Opera, you will need to enable Developer Tools by checking the option in More Tools > Enable developer tools.
Since Firebug is an extension for Firefox, you will need to install the add on from http://getfirebug.com/