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.

Navigate to about:config (in Firefox’s URL bar)
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:

Sam McLeod

A quick tip for disabling scroll-wheel zoom in Firefox without any addons. Navigate to about:config and set mousewheel.with_control.action and mousewheel.with_meta.action to 0. For users with Firefox Sync, two additional boolean properties can be created to sync these settings across machines.

Disabling scroll-wheel zoom in Firefox