I'm in the Chrome camp, but for the times that I've used Opera's Dragonfly thingy, I've gotten value from it. The only thing that used to suck about it was requiring an Internet connection. A few years ago when I was in varsity Internet was a luxury outside campus.
debugger is a nice keyword, although I've noticed it slipping into committed code (at work) more and more frequently. Our code review process catches it most of the time, but it's noise in the source control system (another commit to remove it).<p>Doesn't make it any less valuable, and a stray debugger; statement doesn't cause any problems out in production.<p>The last three were new to me. Emulating mobile layouts will save a heap of time, currently I use Electric Plum's simulators (which are good, but another tool to set up).
Related, more good tips: <a href="https://plus.google.com/+MarkBennettCa/posts/N1qwzwuAZd1" rel="nofollow">https://plus.google.com/+MarkBennettCa/posts/N1qwzwuAZd1</a>