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?

Menu-Tools-Extensions
Left click Image Zoom (highlight it) bottom of extensions window Options

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