Screenshot: http://grab.by/4X87 The resizer should appear mirrored and on the bottom left corner.
Markus, looking at the patch landed as part of bug 514745, this should work correctly, but it doesn't. Do you have any idea why?
As I said in bug 554810 comment 39: Window resizers are special. We don't control them, they show up as a result of setting the "resizable" flag on the Cocoa window we create. They're not themed by -moz-appearance: resizer. (That's also why they've always been transparent.) I don't think we can fix this, other than declaring our windows as non-resizable and implementing the resizers ourselves. But that would mean that every resizable window needs a <resizer> in its XUL, which is not the case. On Windows and Linux that doesn't matter because on those platforms you can resize windows at the window border.
OK, this is unfortunate, but so are a lot of things on Mac. :( ->WONTFIX.