How to run an app in a framebuffer?

The general answer is: you can not. Framebuffer is a different (you can say: more "basic") way of interfacing the graphics than an X server creates. Only the apps that where designed to utilize a framebuffer are able to do it. And there aren't many graphical apps which contain such support - the framebuffer is mostly used for text-mode (console) applications. Firefox is a classic example of an app that was designed to run on top of an Xorg server (just as most of the grpahical apps).

However, if you are really interested, there are some projects that use the framebuffer as base for a bit more advanced graphical apps. Probably the most advanced can be found under the DirectFB project page. This actually does contain some information about running Firefox in framebuffer mode (that is, under DirectFB environment). Notice however that it is only an experimental port of Firefox - very old and apparently abandoned around 07-2008.