I changed the 'mousewheel.with_shift.action' to a '5' in 'about:config' to be able to use pinch zoom on my desktop with a keyboard and mouse, like I'm able to do on my phone or laptop with a touch screen. This does work properly and I'm able to pinch zoom by holding down the Shift key and scrolling my mouse wheel up and down.

However, there is an issue of sensitivity. The pinch zoom is painfully slow, and I have been unable to adjust its speed. If there exists an option to change the pinch zoom sensitivity, I'd greatly appreciate the help.