From the sound of it, Opera Software is working on a development suite that will have the combined functionalities of the popular Web Developer and Firebug extensions for Firefox, and probably more. Given the low memory footprint of the Opera browser and its usage in mobile devices, we can expect that Dragonfly will perform well in the mobile web development department.
That won't be all: "Opera Dragonfly is a new breed of hybrid application. Part desktop application, part web application, it resides in local persistent storage, yet instantly updates when a new version is released – just like your favorite web sites. You never have to check for updates or install a new version."
Opera is free to download and use, but it is closed source. Dragonfly, on the other hand, will be free as in both "beer" and "speech," being licensed under the BSD license.