The diagram on the right illustrates how things everywhere are. Everywhere except Firefox, that is. For reasons unknown to me Firefox does the exact opposite of what would have been intuitive. I.e. increase the font size with up and decrease it with down. Firefox, however, does it the other way around.
I'm using Firefox for years and I still mix that up over and over again. Today was the first day I became aware of that though. After failing to increase the font size I stopped reading, looked at the mouse wheel, and wondered why it didn't increase the font size. I moved the mouse wheel up, didn't I?
I moved it up indeed, but Firefox didn't meet my expectations. "Up" equals "more" and "forward" means closer to the screen, which also equals bigger font sizes. No matter how I look at it there doesn't seem to be any explanation for the observed behavior.
Fortunately there is a way to undo this inversion: change the value of mousewheel.withcontrolkey.numlines (over in about:config) to "-1". After doing that Firefox behaves like any other program. Great. Well, the name of that variable doesn't make much sense, but it really is the right one.
I wonder why they are shipping Firefox with this nonsense default behavior. It really doesn't make any sense, does it? Ah well, at least it's easy to fix.