Did you also look here:

https://www-archive.mozilla.org/docs/end-user/moz_shortcuts.html

Hi kenamoru, you know, I never noticed that the "two fingers spread apart" gesture on the trackpad zoomed the page. Is that new?

It makes sense to me that those things move together: when the zoom bubble in the address bar reads 133%, the page content should be 133% larger than its starting size.

Maybe Firefox didn't pay attention to the gesture before and Windows did the magnifying trick. If that's a recent change, maybe there is a way to set Firefox to ignore trackpad gestures if that's what it used to do. Hopefully someone has time to research that today.