Like firefox has firebug as a efficient tool for web application developer, I am wondering is there any similar tool in opera which is for web developers?
In Opera you can use DragonFly
Opera Dragonfly 1.0 is included by default with the Opera browser. Launch Opera Dragonfly with Ctrl + Shift + I on Windows and Linux, or ⌘ + ⌥ + I on Mac. Alternatively, target a specific element by right clicking in the page and selecting "Inspect Element".
http://www.opera.com/dragonfly/
A video on youtube http://www.youtube.com/v/BeTzJ-HGLo4?version=3&modestbranding=1&rel=0
Opera Dragonfly is a cross device, cross platform debugging environment for the Opera browser-debug JavaScript, inspect and edit CSS and the DOM
More Info http://www.opera.com/dragonfly/documentation/
Alternatively you can also use Firebug Lite in Opera