Remove titlebar from window like Vimperator

There's a plugin for Firefox called Vimperator which is what it sounds like — a vim style interface for Firefox. It has the curious ability to nearly remove the titlebar of the Firefox window. This functionality is accessed with vim-style commands.

Default Firefox

set gui=all

Firefox with almost no titlebar

set gui=none

Does this depend on something special about Firefox or is it possible to apply this change to apps in general or maybe to apps that use a particular framework?

By possible, what I really mean is possible without modifying the source code of the program.