Bug 879981 somehow fixed the issue that the urlbar/back button couple is not removable. But in doing so, it made it impossible to move at all within the navbar, while it actually is possible to move it... if you move every other element accordingly (which is tedious).

Bug 879981 comment 35 is correct: "This was the design decision made here. Short of backing out the entire fix, or going with a completely different fix, I don't see what could be done about it. Pretending you could move it anywhere and then snapping it back to the navbar was also bad UX - that's why we've done what we've done. You could file a followup bug to only allow dragging the item within the confines of the navbar, but that's very hard to fix, and won't be done for 29 (or probably even 30)." The originally-desired UI here was to allow dragging non-removable items, but have them remain locked in the toolbar and just slide left-and-right. But that's Hard To Do, so this is compromise to indicate non-removability. We can leave this open for a better fix in the future, but this won't get fixed for the initial release.

Australis looks cool and brings us some nice customizability options. But it would be even better to be able to move the urlbar/back button couble *independently*. If I am not mistaken, this was possible in the past, wasn't it? Anyhow, thanks for Australis!!