Interesting Firebug story: Years ago when Netscape/Firefox was being built, some developers wanted to put a bunch of developer tools into the browser. Mostly so Firefox developers could inspect the browser (remember, websites were pretty bland back then).
However, they were worried about the codebase becoming too big -- after all, 99.9% of people wouldn't need a DOM inspector. So, the add-ons architecture was born. Firebug was created as a separate tool by Joe Hewitt. Add-ons and Firebug probably ended up being the reason why Firefox took off.