Oops ... almost perfect. There seems to be just a very minor issue:
In Firefox dev and Firefox 37.0 (but not Waterfox), the titlebar sometimes loses transparency. It seems to happen when switching to Firefox from another application, sometimes when moving the window, and sometimes when the mouse has been over the titlebar and then moves away. The titlebar doesn't go completely opaque, but becomes about half as transparent as it should be. When it happens, there's often a hard black shadow behind the top of the window, except for where the window controls are.
Here's a screenshot showing the issue with the Acrylic skin.
At least it's usable! Still looks much better than raw Windows 8.1.