I develop using firefox/ firebug. I do really like the way firebug works, but its starting to show its age a bit.

The integrated tool are getting better but I often forget they're there. The responsive page resizer/viewer I use frequently

Tools->web developer seems to incorporate firebug and the native tools (confusing...). I like the way firebug puts a little button on the menu bar I can click to turn debuggin on and off.

We'll see how this shakes out.