Start mozilla. Dismiss the sidebar by clicking on the grippy. A 100x100 (maybe 200x200) area bounded by horizontal/vertical scrollbars is displayed in the upper left cornerof the screen. Presumably these are the scrollbars that bounded the sidebar, that need to be unmapped.
box bug. I see this on Linux too. On windows I see a little grey box in the upper left.
*** This bug has been marked as a duplicate of 13876 ***
VERIFIED as a DUPLICATE.