From bug 605622, comment 4: "... ... Ideally the top border would actually be 1px solid ThreeDHighlight; (same as a regular toolbar) (in all three windows) although the existing -moz-border-top-colors makes this a little trickier." (this is for win/linux)
Created attachment 489238 [details] [diff] [review] Fix top border OK, so it would have been cleaner with the reversed colour sequence in win-/gnomestripe, then one could just had set a width of 1px...
(not tested on win/nix)
Comment on attachment 489238 [details] [diff] [review] Fix top border Nit: you don't actually need to duplicate the colour, as the border is only 1px.
Pushed with nit addressed: http://hg.mozilla.org/comm-central/rev/5d48dda12543