A big part of the built-in tools isn't to replace Firebug. It's to provide APIs for Firebug (and other addons!) to be able to do what they do even better. Add

A big part of the built-in tools isn't to replace Firebug. It's to provide APIs for Firebug (and other addons!) to be able to do what they do even better. Addons are loading arbitrary code into your browser though, so there is a potential for perf impact. We're looking at that too, with out-of-process addons (see https://addons.mozilla.org/en-US/developers/docs/sdk/1.3/dev-guide/module-development/e10s.html https://addons.mozilla.org/en-US/developers/docs/sdk/1.3/dev...). Addons don't in general hurt perf, but it is definitely possible for them to. Firebug, in particular, disables the JIT so it can debug it, making Javascript execution painfully slow, and thus the browser is slow too, since large parts are written in JS. Firebug isn't to blame though; we just haven't provided them with APIs to do better yet.