In a 64-bit C# window forms app can I use geckofx or there is no such a solution? Thanks
You need GeckoFX source code. Build in x64. Then download xulrunner source code from ftp.mozila.org (same version with gecko fx) and build in x64.
An alternative to building GeckoFX from source is to use CorFlags.exe to mark the assemblies as 64bit (by Tom)