Firebug in this context is a Firefox addon from here which aids in debugging and correcting Javascript code and CSS layouts and XML.
With it installed into Firefox you can
Inspect and edit HTML (changing it on the fly to see effect)
Tweak CSS – again make on the fly changes to get what you want before changing your code
Visualise CSS issues – fix those problems with padding and borders to get what you want
Monitor network activity – whats taking time to load on the pages
Debug and profile javascript – see whats running and for how long – tune your code
Explore the DOM – see inside the Document Object Model
These items only scratch the surface of this tool and I look forward to its insight in xpages and other web development I have in mind.
PS – there is a “lite” version available for other browsers i.e non firefox