Firebug is like steel so heavy to use. Once I have accumlated enough logs the tool just starts lagging. Devtool is awesome but definitely still lack of some nice touch. I think there are still so much to improve with browser-side devtool. One annoying thing is (regardless of which browser, which devtool), editing stuff always happen in a small portion of a screen. If we can have a dedicated devtool app that'd be awesome. Bigger font, more tabs, drag and drop placement / customized view), and with extension (You should be able to do that already, don't remember).

Sure you can drag the devtool out, but it still lives as part of the browser windows, and switching back and forth between several instances of browser windows? tough! I am not sure, but has Mozilla hired the folks behind Firebug already?