In Firefox 49.0.1 on my PC the "Upload a File" image is in a completely white box but shows up fine on my android phone running Firefox 4.4. Also note that with Ad Block Plus enabled, it also shows a "Block" icon above when I do a mouse over in the white space which doesn't show with ABP disabled or on any other of the selections next to it. See attachments for details. It just started showing this within the last few hours or so. The button still works though as that is how I uploaded the 2 images to this post.