After further looking into this, I’m not sure this is even possible. One of the reasons I disliked using chrome in Linux, is because it was extremely intrusive, and did not integrate well with my system’s window manager, especially if the window manager I was using was not the distribution’s default.
I’m pretty sure that scroll bars on application windows are implemented at this level of the operating system, and such a feature would require finding a way to override the system’s scroll-bar for every window manager that Firefox supports. Not only would this require a massive amount of effort, but it would also frustrate users who like their scroll bar to match on all of their application windows (such as myself).
Just for a frame of reference, here is a list of window managers that Arch Linux supports: https://wiki.archlinux.org/index.php/Window_manager