StormDriver writes "Firefox 15 has hit the Mozilla pre-beta Aurora channel, and it features a redesigned, built-in debugger."
The original weblog post
has more. Thanks to improved debugger internals
in SpiderMonkey, supposedly code should run just as fast with debugging enabled as without (ever try loading Slashdot with firebug accidentally enabled?). There are also new tools for testing mobile layouts from the comfort of your workstation, and the debugger can attach to remote processes
(Something Emacs users have enjoyed for years now
, albeit in a hackish manner and without support for mobile Firefox).