I believe this is linked to Firefox's setting for the "mouse wheel". Check out the following thread and see whether that helps: control key (ctrl) and scrolling issue (for Windows 8, but same principle).

Nope, didn't change anything

Sorry, after a little more research, I think on MacOS the relevant setting is for the "meta" key not the "ctrl" key.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste meta and pause while the list is filtered

(3) Double-click mousewheel.with_meta.action and enter the desired value from the following list, then click OK.

0: Nothing happens
1: Scrolling contents
2: Go back or go forward, in your history
3: Zoom in or out

Any luck?

Yes!

Thank you! Have a internetcookie on me :)

Can I just change this shortcut ? For example - scroll zooming + cmd + shift or another button, I like this option but not always.

You can modify the action for any modifier key that you see on the about:config page.

mousewheel.default.action
mousewheel.with_alt.action
mousewheel.with_control.action
mousewheel.with_meta.action
mousewheel.with_shift.action
mousewheel.with_win.action

See also:

• resource:///defaults/preferences/firefox.js
• http://wiki.mozilla.org/Gecko:Mouse_Wheel_Scrolling

0: Nothing happens
1: Scrolling contents
2: Go back or go forward, in your history
3: Zoom in or out

Thanks jscher2000, I've been trying to figure out how to disable the mouse scroll wheel zoom in Firefox. Your tip worked like a charm.