Every application talks to the Display Server, calling repaints and everything, but those are not kernel messages/kernel mode switches, that is user mode communication. Inital Modesetting is one of the few things that the X-Server did in Kernel Mode(Thus requiring root rights), but we are moving away from that.
D-Bus however is Daemon for Applications to talk to each other over this protocol instead of the crazy way of shared memory. So if, let's say Amarok changes the song played it calls out a D-Bus Message, and the Now Listening-Plugin of your instant messenger will pick it up. The daemon for D-Bus resides in User-Space, but messages between processes happen in Kernel-Space, so you have a context switch every time a message is send.
Firefox communicating with X does not happen in Kernel-Mode. Sure your drivers may actually do some Hardware calls in Kernel-Mode, but once again, pushing Firefox into the kernel will not make it faster(or may make it faster due to other factors, threading and stuff like that, but can you spell security nightmare?)
So yeah, D-Bus' only purpose makes him actively use Kernel-Space, and it's a cornerstone of the modern fs-ecosystem so why not merge it there?