The close icon is mispositioned because the close-icon class uses a bigger close icon on win8 and win10.
Created attachment 8660270 [details] [diff] [review] sidebarClose.patch This is the same simple change as for bug 1192999 with using close-XPVista7.png instead close.png. Comment 0 is wrong it's not because of the close-icon class, it's becaus close.png is the big icon on win8 and win10 and on the other through override in manifest close-XPVista7.png is used. With using close-XPVista7.png all the time this is clear which icon is used.
https://hg.mozilla.org/comm-central/rev/bf80a5bc39d2f702a7a522e5b65d6e7d562a4326 Bug 1204206 - Use close-XPVista7.png for the contacts sidebar close button. r=clokep a=aleth