For me Firebug seems ~2x as slow (eg, for a 1s operation in native tools, firebug takes 2s). Opening Firebug on my workstation causes a noticable pause in all activity on page for several seconds, while opening the native tools only takes about half a second. All of Firefox's dev tools are still a ways from being as fast and bug-free as the Webkit tools (both Chrome and Safari). I can't cite specific examples since I dropped Firebug in Firefox once the native tools reached a certain level of quality. I just suggest trying to use 'em and seeing if you like them more than Firebug. I did.