penders - 5:14 pm on Sep 27, 2011 (gmt 0)
Firebug is Firefox Add-On. So, using Firefox, navigate to the firebug page at mozilla.org....
And click "Add to Firefox", and follow the instructions.
There are similar tools built in to Google Chrome (right mouse and select "Inspect Element" to access the debugger). And in Opera there is Opera Dragonfly (again, right mouse and select "Inspect Element"). IE8 has the Developer Tools (F12).