Is Firefox 3 a Cocoa app on mac?

Well, yes and no. Firefox 3 uses Cocoa Widgets. Josh Aas went into more detail in Firefox 3 for Mac OS X: Under the Hood, I'll provide a relevant excerpt:

There are only 2 types of Cocoa objects at the heart of Gecko 1.9 on Mac OS X (let’s forget about the menu bar at the top of the screen for now). Cocoa’s NSWindow allows us to make and control a window on the screen and Cocoa’s NSView allows us to draw things into a window. Those two objects also allow us to get most of the events we need to know about from the operating system (such as key and mouse events). We do not use actual Cocoa buttons or any other Cocoa controls within any Gecko 1.9 windows. The context menus, dropdown menus, the toolbar, the search bar, the buttons and text fields within web pages - they are not actual Cocoa controls. For example, instead of using actual Cocoa buttons for “Submit” buttons we just draw the image of an Aqua “Submit” button into an NSView, one of the basic Cocoa objects we use. Gecko 1.9 has Aqua form controls because we now draw images of Aqua form controls when appropriate, not because we use actual Cocoa controls. The reason we don’t use actual Cocoa controls isn’t because we are lazy or we can’t figure out how to use them or because we are constrained by our cross-platform requirements - Apple’s WebKit doesn’t use actual Cocoa controls for things like “Submit” buttons or popup buttons in web pages either, at least not the last time I checked. IE for Windows is in the same boat. The reason Gecko 1.9 doesn’t use Cocoa controls is for the sake of flexibility - form control behavior and appearance can be changed significantly via HTML, CSS, and JavaScript. Actual Cocoa controls are simply not flexible enough to do all of the things that people want to be able to do with controls on the web.

So if we didn’t get Aqua form controls out of the deal why did we do the Cocoa rewrite? First of all, Apple has deprecated much of Carbon already and has made it clear that Cocoa is the future for Mac OS X applications. Apple is investing heavily in Cocoa, which benefits us if we use Cocoa. Cocoa also gives us access to great features that would be more difficult or impossible to take advantage of through Carbon. For example, with Cocoa we were able to relatively easily draw using CoreGraphics instead of the ancient Quickdraw API (more on this later). Secondly, the Cocoa way of doing things matches up with the Gecko way of doing things better than Carbon did. Our Cocoa code is easier to understand and maintain because of this. This will result in faster development and fewer bugs in the long run. In fact, we actually added more capabilities to Firefox 3’s Gecko 1.9 (such as transparent windows, unified toolbar windows, and an alert service) than we have ever added in any release of Gecko since Firefox 1.0. This is in addition to doing a huge amount of work just to re-implement many of the features that Gecko 1.8.1 had already implemented in Carbon!

However, the dictionary invoking with Command+Control+D is bug 301451. You are free to use the vote feature to show support for the bug, but please refrain from commenting with a "me too" like statement.