I find I have to fish around in each app for size controls. Even in Windows 8 this is not mandated that all apps follow the system settings. Not much has changed yet.
In firefox try the old Control + and - to size up the display in the browser. The Windows setting you wrote about should increase menus but not what's inside the web browser frame.
I just got a Samsung ativ book9, and as much as I do love it, EVERYTHING on my screen is tiny! The first thing I did was go to the ease of access center and switch on the "make everything on my screen bigger" button, which was fine for the home screen but on my desktop everything was still little.
So I looked into it a little more and discovered that if I went to desktop and made a custom font size, everything would get bigger! I went ahead and did that...but now the home screen is little again, and only SOME things got bigger(firefox in general, my desktop icons). Still, when I try running a program in cmd, I can barely see what I'm typing. Also, with such things as spotify, I can't even exit out because the exit symbol is too little. Lastly, on youtube and netflix, the buttons are still so little that they too are hard to click on.
How can I make it so that everything on my computer is human sized?