Thanks, @JoshStrobl . Looks like I'm still seeing the same issue on the updated v148 r2.
Jul 31 2018
Apr 13 2018
Oct 8 2017
+1, after making that change and rebooting, the Mad Max Vulkan beta branch is running for me. (I know this is technically about Dota 2, but...)
Jun 16 2017
I see this behavior with all three Arc themes (Arc Theme, Ark Darker Theme, Arc Dark Theme) as well as the Compact Dark and Compact Light themes. I do not see anything wrong in the downloads pop-up using the default Firefox theme: http://i.imgur.com/3VAXYqn.png