Firebug slows down start-up time of Firefox. It is at the second place in the list of top 10 slow performing add-ons. Well, the newly introduced feature “Delayed Loading” improves start time of Firefox by loading Firebug only when the user launches it.
Moreover, Firebug 1.10a2 is now a restartless add-on, which means that it can be installed, removed, enabled or disabled without restarting the browser. Firebug team is also working on implementing restartless feature for Firebug extensions.