Thrown by scroll inertia in Firefox
I've discovered a rather annoying quirk in the combined environment of OS X 10.6 (Snow Leopard) and Firefox. It seems that the Mac thinks a "thrown" scroll lasts much longer than you might imagine. I'm referring to the inertia feature when using a two-finger scroll gesture on a touch pad. If you push or pull at a decent speed and then lift off, it'll keep scrolling, but will gradually slow down.
Now here's the problem. Let's say you've now hit the top or bottom of some web page. Maybe you go to type something or do something else which involves the control key. As soon as you push that button down, the remaining inertia (which you didn't even realize was there) turns into a zoom command. Instantly, you get either massive screen-filling characters or tiny Flyspeck 3 stuff.
What's particularly frustrating about this is that you can't reproduce it just by holding down control and then doing a two-finger zoom. If you do that, the OS itself grabs it and does a "zoom-the-whole-screen" change instead. The only way to pipe this through to Firefox is to have it doing the inertia thing and then push Control. It took me a bit to figure this out.
It looks like the solution is to set mousewheel.withcontrolkey.action to 0. It's unlikely you're already using it given the OS default of "wheel" + control for whole-screen scrolling.