This is very experimental, but since no one else has replied...

I'm not sure which computers are affected, but Firefox has some support for touchpad "gestures" on multi-touch touchpads. Most of the ones that affect the zoom level of the page are the "pinching" gestures, like a smartphone screen. However, it seems that by default a "tap" will reset the zoom level, and that might be the problem here.

Could you try this?

(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 gest and pause while the list is filtered

(3) Double-click browser.gesture.tap and change it to... I'm not sure whether there is a command for "do nothing" but you could try typing or pasting in cmd_selectNone which cancels the current text selection in the page (if any).

I'm not sure if you can change the page zoom when scrolling and the Ctrl key is pressed like you do with a mouse (Ctrl + rotating scroll wheel).

I have had this problem on my Acer Windows 8 laptop, and now that I know about the right part of about.config I'm trying altering the browser.gesture.pinch.threshold value to be less sensitive (I've seen 150 recommended).