Opera was my secret weapon back in the day: if it worked in Opera, it would be guaranteed to work in Chrome, IE and Firefox. It significantly reduced the browser quirks stuff I'd have to dig into.

Dragonfly was top notch also: one of the best bits was ability to outline all the elements on the page. There were other features too that weren't (still aren't) in the other browser dev tools

Huh? If it worked in Opera, it absolutely wasn’t guaranteed to work in IE < 9 (conservatively; honestly probably every pre-Edge IE). At one point Opera had a more faithful CSS (2) implementation than even Firefox. And nothing guaranteed it worked in IE except checking in IE.