Created attachment 8781492 [details] 2016-08-16.png User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0 Build ID: 20160604131506 Steps to reproduce: This happens for me on Windows 10 and OpenSUSE Tumbleweed (KDE/Plasma 5). Every time I download a file, the animation leaves behind an artifact on the toolbar that exists until I restart Firefox (or do another action that causes the toolbar to redraw, which I have yet to identifiy).
Happens on branch 48 as well (in KDE at least).
Confirmed on branch 48 on Windows too, however now you can 'clean' it up by waving the mouse over the affected area of the toolbar, which you could not do before IIRC (although I'm not about to downgrade to check).
it's fixed in 49