I use Chrome or Firefox built in developer tools, however I still think Firebug fits my workflow the best. Unfortunately it's too slow to use. The thing I miss the most is the inline display of AJAX call responses, and being able to write/run multi-line snippets of JavaScript (without ctrl+enter). Somewhat interestingly, Firefox built their entire add-on architecture because Joe Hewitt wanted dev tools (for debugging…
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.