If I can see a movie from the console (like in this post), then how can I use other apps like Firefox from the console? I'm looking for something that works in Ubuntu, Fedora, or OpenBSD.
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.
SDL is a notable library that supports the framebuffer.
And with a library like AntTweakBar, you could in theory write any UI in it (though I suspect performance will be hard to optimize).
So people have already done an SDL-based browser that runs on the framebuffer: http://www.netsurf-browser.org/about/
There is an Ubuntu package for it, but I haven't managed to make it work yet: https://askubuntu.com/questions/817937/how-to-run-netsurf-fb-fails-with-unable-to-set-video-could-not-set-console-s
vlc is another interesting software that works on the framebuffer: https://askubuntu.com/a/550014/52975 and that did work for me on Ubuntu 16.04.
This answer has a good analysis of dependencies: https://unix.stackexchange.com/a/177209/32558 and also points to this guy's page: http://litcave.rudi.ir/ which has some more hardcore fbdev software like a PDF reader.
GTK+ seems to have a framebuffer fork: http://www.gtk.org/api/2.6/gtk/gtk-framebuffer.html and Qt official support: http://doc.qt.io/qt-4.8/qvfb.html (it was used by Nokia in embedded).