Firefox in 2011 – Firefox plans for 2012

Firebug killed performance because it disabled JIT, otherwise it couldn't do its work. If you don't know what JIT is, it basically converts the javascript to bytecode 'just in time' for execution. Commonly executed javascript doesn't need to be reinterpreted, giving speed improvements. It also requires a lot more memory. I'm not too sure how the new firefox web developer tools and chrome developer tools do, I assume they turn off JIT just for the current page, whereas Firebug was global.