See testcase, no matter what width the block has, when you dotted borders, there always seems to be a bit of chaos at the corners where there can be up to 3 dots crammed to each other. Branch builds handle this nicer (although they don't really have circles as dots). I've compared this with other browsers, IE8 seems to handle this case the best.

This is essentially the same bug as https://bugzilla.mozilla.org/show_bug.cgi?id=19963. That bug has the highest priority of all dotted-border-corners-ugly related bugs. It is also the oldest and it has a (seemingly active) developer assigned to it. I suggest to "merge" this bug with 19963. If you care, please check it out and vote for it.