To install Firebug, you first open the main Firefox menu from the lab in the top left-hand corner of the browser.
You then select Add-ons from the menu. This brings up a new wenpage for the Add-ons Manager.
Type "Firebug" in the Search box at the top left of the Add-ons Manager webpage.
This will bring up a webpage with many results - you will need to go the bottom of this page, and press a button so to show all the search results.
The first result that shows up in the "Show all results" search should be the Firebug application.
You can then choose the option "+ Add to Firefox" to install Firebug, after which you must restart the browser.