Firebug 2.0
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.<p>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).<p>Somewhat interestingly, Firefox built their entire add-on architecture because Joe Hewitt wanted dev tools (for debugging the actual browser moreso than websites), and Firebug was split off from Firefox and made into an add-on. A decade later, this has reversed and every major browser now ships with integrated dev tools.