Firebug 1.x (only with activated "Scripts" tab) is only slow in newer version of Firefox. http://www.reddit.com/r/firefox/comments/1f4u9x/firebug_make...

I read that Firebug 2 now uses the newer JSDBG2 API of recent Firefox, so there should be no performance problems as there were with Firebug 1.x in recent Firefox versions.