I just saw that Opera will stop using Presto, and switches to WebKit, so will DragonFly still exists, or this will be replaced by Chrome Inspector?
And what about Ex
There will be no Dragonfly in Opera with WebKit (Blink): https://twitter.com/runeh/status/301616059729969152
Yes, Opera Dragonfly will still exist. The current beta of Opera 15 includes Web Inspector, however, Opera are porting Opera Dragonfly to Blink. It will be included when ready.
Note: I was formally the Product Manager of Opera Dragonfly, before switching companies.
Opera Extensions use a different format for the Blink based Opera. It uses the subset of the Chrome extension model. There is a tool to convert the existing extensions, and documentation: http://dev.opera.com/extension-docs/