Aargh! Why is my mouse-wheel doing this (probably Firefox-related)?

Randomly when I’m surfing (I use Firefox), if my mouse is over an image and I use my mouse-wheel to scroll down the page, instead any image that the mouse happens to be hovering over increases (or decreases, depending upon the direction) in size.

Annoying, especially on image-heavy sites (including this one).

Doesn’t happen all the time, but enough to be a royal pain.

I’m assuming it has something to do with a Firefox extension - I do have Image Zoom installed, but it has no options related to mouse zoom.

Help? How to disable this function?

Left click Image Zoom (highlight it) bottom of extensions window Options

Are you seeing a magnifying glass when you mouse over pics?