User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:32.0) Gecko/20100101 Firefox/32.0 SeaMonkey/2.29 Build ID: 20140904220358 Steps to reproduce: I open a web page or email message that's longer than a single screen and scroll through it with either the scroll bar or my mousewheel. Actual results: The scroll bar slider disappears. Expected results: The scroll bar slider should be there for me to grab.

Comment on attachment 8487273 [details] Capture.PNG The slider also disappears when I click in the scrollbar.

Confirming this bug with Mozilla/5.0 (Windows NT 6.1; rv:32.0) Gecko/20100101 Firefox/32.0 SeaMonkey/2.29 (PortableApps version). When opening a web page, the scroll bar slider is visible, but when clicking on it, it disappears and remains invisible. Also dragging it from where it should be does not work, so it seems to be really gone.

Seemingly should be a Core issue, as FF is likewise affected. See, http://forums.mozillazine.org/viewtopic.php?f=9&t=2865431

Bug 1066934 - FF32: Skinned scrollbars thumb disappear upon scrolling a page at specific zoom percentage (different between FF themes)