Smooth CSS transition for FxA popup (bug 1074348)

https://bugzilla.mozilla.org/show_bug.cgi?id=1074348

No gifs for this one because they drop the unstyled content flash.

Before: https://www.dropbox.com/s/t1c9fbf5nzw7s9j/reload-before.mov?dl=0
After: https://www.dropbox.com/s/2hvmvqrmbau415c/reload-after.mov?dl=0

I don't want to be specific in case the layout changes. So I think that pretty much means it needs to be `!important`.