Fire is written in 100% Elements code, most of it C# (for a few reasons. For one, I needed a good real live dog fooding project for the C# compiler! for another, C# was easier to write back when IDE support for stuff such as Class Completion wasn't ready yet). The core app is all native Cocoa.
Internally, the app hosts the compiler and other "IDE smarts" infrastructure that's written in .NET and shared with Visual Studio. Most of the codebase is written in Oxygene, and we host it via a glue layer called Marzipan (we've open sourced that and it's on github) to make it easy and fast for the Cocoa code to interact with the managed code.