Finally I got this “feature” to work the way I really want, page-zoom on mousewheel is done on my laptop!
The problem is:
Ctrl + something is a widely used combination on keyboard shourtcut,
Ctrl + Tab,
Ctrl + Shift + Tab,
Ctrl + +,
Ctrl + -, etc etc…; another way to trigger the zoom on Firefox is
Ctrl + mousewheel, unfortunately the mousewheel gets remapped on touchpads as two-finger scroll, that way you can scroll quite nicely, too bad that if I press enter the zoom comes into play and
Ctrl + 0 is the only way out to normality.
So on to the solution, open about:config on your Firefox, dismiss the safety notice and search for mousewheel.with_control, a nice list of options pop out.
The default value for mousewheel.with_control.action is a string with value 3 which in some way maps to zoom, set that value to 1 and enjoy your new experience, no more unexpected zoom when hitting Ctrl for something else to be done.