User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20170823100553 Steps to reproduce: Place the download button behind the ">>" addon menu. Update to Nightly 8/23/2017 4:30PM PST update. The Download button disappears. It's not found under Customize either. Download any file. The Download button appears automatically and places itself on the right side of the address bar (not within). Moving or removing it from Customize is impossible; it's grayed out and attached to the address bar. Click on Download button to open the small popup list, and clear the entire history of downloaded files. The Download button will disappear again. It was working properly yesterday while this bug was reported https://bugzilla.mozilla.org/show_bug.cgi?id=1392633 So this appears to be a different bug.