Bug 1676149 - Set a pref that a test relies on.

Differential Revision: https://phabricator.services.mozilla.com/D96413

dom/events/test/test_dom_wheel_event.html

@@ -804,6 +804,8 @@ function runTest()
     // something else.
     ["layout.css.devPixelsPerPx", 1.0],
 
+    ["dom.event.wheel-deltaMode-lines.disabled", true],
+
     ["mousewheel.default.delta_multiplier_x", 100],
     ["mousewheel.default.delta_multiplier_y", 100],
     ["mousewheel.default.delta_multiplier_z", 100],