In many cases the arrow of arrowpanels doesn't point to the center of its anchor. It should always be attached to the middle of the button. On UX builds for example put History widget to the right of the bookmarks bar and open it (with browser window maximized). The Panel and the panel arrow appear too much on the left. Note that it isn't only Australis related, but it would be a good time to resolve this.

Does bug 812943 help here? A screenshot would be useful.

Here is a screenshot of three cases : First one is when the window is a bit off-screen with a classic panel. The arrow is wrongly placed. Second one is the new history widget on the right of the bookmarks bar after being customized. The arrow is out of place too. Third one is the new panel menu when the window is partly off-screen. Strangely it's the only Firefox arrowpanel that behaves correctly since the arrow is attached to the arrowpanel no matter what (what Google Chrome currently does):

I forgot to add that menu panel arrow is also out of place when the window is a bit too off-screen.

The panel code goes out of its way to ensure a popup isn't off-screen. This makes sense for menus, but I agree doesn't make as much sense for arrow panels. I'm not sure what is causing the issue shown in the middle image in that attachment.

Middle image shows a bug with Australis customization mode. It's been resolved since concerning the vertical arrow placement, but horizontally it's still not good.

This is subject to a lot of discussions and it has been resolved for the menu panel in bug 874792 so I'm closing it. It should maybe be revisited in the future.