Possibly regression from bug 763845?
Created attachment 632780 [details]
Nightly (06/13) - Screenshot
Regression from bug 759755.
*** Bug 764292 has been marked as a duplicate of this bug. ***
Might be fixed via bug 764354.
Confirmed fixed via tinderbox-build in bug 764354.
Layout issues were fixed on Firefox 16 Beta 5 and Firefox 15.