This feature annoys me endlessly, I end up zoomed in and out of websites all over the internet. … But the fix is easy and there’s no addons required.

  1. Navigate to about:config (in Firefox’s URL bar)
  2. Change the value of the following two properties to 0:
mousewheel.with_control.action
mousewheel.with_meta.action

If you use Firefox sync and want these settings to sync between your machines, also add the following properties:

Create two new properties both of type boolean and set them to true:

services.sync.prefs.sync.mousewheel.with_meta.action
services.sync.prefs.sync.mousewheel.with_control.action

Enjoy!