User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0 Build ID: 20171210220040 Steps to reproduce: I can only reproduce this on linux nightly (both old and new profiles), Windows nightly does work as expected as well as Firefox quantum. Steps to Reproduce: Hold mouse still and click middle mouse button (autoscroll) on page e.g. mozilla.org Actual results: Autoscroll starts scrolling downwards as soon as you middle click on page Expected results: Autoscroll should remain still as long as you dont move mouse up or down